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Operator
Ladies and gentlemen, thank you for standing by, and welcome to Altair Engineering, Inc. Fourth Quarter 2020 Earnings Conference Call. (Operator Instructions). Please be advised that today's conference is being recorded. (Operator Instructions)
I would like to hand the conference over to our Chief Financial Officer, Howard Morof.
Howard N. Morof - CFO
Good morning. Welcome, and thank you for attending Altair's earnings conference call for the fourth quarter of 2020. I'm Howard Morof, Chief Financial Officer of Altair. And with me on the call is Jim Scapa, our Founder, Chairman and CEO; and Matt Brown, who assumed the CFO role on March 16.
After market closed yesterday, we issued a press release with details regarding our fourth quarter performance and guidance for Q1 and the full year 2021, which can be accessed on the Investor Relations section of our website at investor.altair.com. This call is being recorded, and a replay will be available on the IR section of our website following the conclusion of this call.
During today's call, we will make statements related to our business that may be considered forward-looking under federal securities laws. These statements reflect our views only as of today and should not be considered representative of our views as of any subsequent date. We disclaim any obligation to update any forward-looking statements or outlook. These statements are subject to a variety of risks and uncertainties that could cause actual results to differ materially from our expectations. These risks are summarized in the press release that we issued yesterday. For a further discussion of the material risks and other important factors that could affect our actual results, please refer to those contained in our quarterly and annual reports filed with the SEC as well as other documents that we have filed or may file from time to time.
During the course of today's call, we will refer to certain non-GAAP financial measures. A reconciliation of GAAP to non-GAAP measures is included in our press release. Finally, at times in our prepared comments or responses to your questions, we may offer metrics that are incremental to our usual presentation to provide greater insight into the dynamics of our business or our quarterly results. Please be advised that we may or may not continue to provide this additional detail in the future.
With that, let me turn the call over to Jim for his prepared remarks. Jim?
James R. Scapa - Founder, Chairman & CEO
Thank you, Howard, and welcome to everyone on the call. Altair had an excellent fourth quarter and full year 2020, especially given the global COVID-19 pandemic and economic uncertainty. I am proud of what our team accomplished in 2020.
In a year of business disruptions and personal challenges, Altaire brought to market, broad and deep additions and enhancements to our product portfolio while delivering solid financial results.
Though some challenges remain due to the pandemic, 2020 has positioned us well for 2021 as we demonstrate successes around our vision for the convergence of simulation, high-performance computing and artificial intelligence, we will continue supporting our customers with industry-leading technology and unparalleled engineering and data science expertise. We have never been in such an energized position regarding our software offerings. Our integrated suite of software optimizes design performance across multiple disciplines, encompassing structures, motion, fluids, thermal management, electromagnetics, system modeling and embedded systems, while also providing AI solutions and true to life visualization and rendering. Our HPC solutions maximize the efficient utilization of complex compute resources and streamline the workflow management of compute-intensive tasks for applications, including AI modeling and simulation and visualization.
Our data analytics and AI products include data preparation, data science and visualization solutions that fuel engineering, scientific and business decisions. We are pleased to report Q4 results with total revenue of $133.4 million. Software product revenue for the quarter was $113.6 million, reflecting year-on-year growth of 12%. Adjusted EBITDA was $21.7 million, an increase of 70% on the fourth quarter of 2019. All were well above our guidance ranges.
For the year 2020, software product revenue grew to $391.7 million from $366.7 million the prior year, an increase of 7%. Total revenue equaled $469.9 million compared to $458.9 million, a small increase, recognizing a continued softness in services. Software product revenue was 85.2% of total revenue for the fourth quarter compared to 81.7% in the prior year period.
For the full year 2020, software product revenue increased to 83.4% of our total revenue from 79.9% in 2019. Our recurring software license rate was 92% for the full year 2020 versus 87% in 2019. Software renewals in the quarter continued to come in as expected with several significant expansions, and new customer activity remained healthy. As we drive the convergence of simulation, HPC and AI into 2021, I feel it might be helpful if this quarter's customer stories focused on momentum toward this convergence.
Panopticon, our solution for real time data streaming and visualization, was selected by a European company specializing in retail customer behavior analysis. Their work includes biometric measures, which can, in turn, lead to predictions around buying preferences. The application supports a multi-format dashboard with data visualization, photos, video and audio. Technical domain knowledge is key in the effective implementation of AI toward engineering applications.
A defense agency is implementing a great example of AI convergence system modeling to support work, including radio signals, satellite communications and electromagnetic spectrum performance. Our work with them started a year ago around electromagnetic simulation. As we help them to bell up a cloud-based real time telemetry data processing, data analytics and archiving system, it will include elements of data acquisition, real time data science and visualization, simulation models and a closed loop and predictive analytics. In the food industry, a large multinational company has been a long time user of Monarch data preparation tools licensed on a traditional named user basis.
After an M&A event, we met with the new parent company and opens conversations around Altair unit's licensing model, especially with regard to the ability of the enterprise to access all of Altair's tools, including data preparation, data science and AI as well as real time data streaming and visualization.
This dialogue turned into a true win-win with the customer now accessing the entire suite of our data analytics and AI tools across many additional users while seeing value in a substantially increased financial commitment.
In a similar fashion, a large financial services organization has been using Monarch for data preparation for more than 20 years. The relationship has been completely restructured from named users on a single application, licensing to a broad deployment of Altair units for all of their data analytics and AI needs.
The result is significantly more users and nice growth in the relationship. We believe the Altair unit's licensing model has potential for disruption in the data analytics and AI market software marketplace. A global leader in agricultural machinery committed to Altair units for the application of AI to engineering with a goal of using digital twins to improve predictive maintenance and reduce the time and cost required for testing. A European manufacturer of metal parts for the automotive industry is using Altair's AI and engineering capabilities to diagnose, predict and reduce production anomalies.
These wins are great examples of Altair's deep engineering knowledge, helping customers to bring the power of AI to their enterprises. One of our larger data analytics deals to close out 2020, a major technology company, inked a 7-figure purchase agreement with initial use cases around payroll optimization. Along with a matching and reconciliation process, the customer will be using Altair for visualization and scoring analysis. Their internal ROI calculations on this implementation are impressive, and we are delighted to see a customer so emphatic about their ability to get an outstanding financial return on our data analytics and AI tools.
A great example of our vision of converging AI with engineering was expressed in the press release we issued last month, discussing our MoU with Rolls-Royce, Germany to collaboratively connect artificial intelligence and engineering to drive business value across Rolls-Royce's engineering, testing and design of aerospace engines.
The collaboration will address a wide variety of use cases, including applying data science with the vast amounts of engineering testing data, which can lead to a significantly reduced number of sensors needed. This single-use case alone has the potential to reduce recurring costs by millions of euros as a pioneer of the convergence of AI and engineering.
Altair is honored to be the technology partner of choice to help Rolls-Royce, Germany make better daily data-driven decisions and transform their business and products. On the topic of acquisition, I am pleased to announce we have acquired flow simulator from GE Aviation. Both simulator is an integrated flow, heat transfer and combustion design software which enables mixed fidelity simulations to optimize machine and systems design.
As organizations are increasingly simulating complex duty cycles, solutions like flow simulator are needed to model an entire system, including rapid iteration, concept modeling and understanding of system simulation and system behavior and virtually anything that encompasses thermal management, bringing flow simulator to the Altair software set will allow us to expand its capabilities in the aerospace market and make it available to new industries, including defense, renewable energy, automotive and electromobility.
In addition to the acquisition, Altair and GE Aviation have signed an MOU to facilitate a higher level of collaboration and establish a long-term strategic partnership. I'm thrilled to strengthen our relationship with GE Aviation, a long-time customer and equally excited about the future of flow simulator as we will bring the simplification of modeling complex thermal systems to new industries. We continue to evolve Altair's organization and processes toward more operational efficiency and believe these changes are resulting in more effective delivery of technology and expertise to our customers.
One example is with our global technical support organization. Infrastructure and organizational changes are allowing us to provide deep knowledge and expertise to customers regardless of geography, even as our technology portfolio expands and broad. In addition to leveraging global capabilities for customer-facing roles, we continue a path of internal digital transformation.
In 2020, Altair like many companies navigating COVID-19, shifted to a remote model for work and communications with customers and coworkers. Going forward, we expect to return to a hybrid model, remote work and traditional office and travel. We believe these changes, while not affecting our external relationships, will play a role toward EBITDA expansion as we scale revenue.
As announced in December, in the December press release, Howard Morof is in the process of transitioning the CFO role to Matt Brown. I am deeply grateful to Howard for his many years of great leadership and guidance for Altair, especially as we made the transition from a privately held company of many decades to an organization ready for an IPO and then to an operating public company.
In the 3 years since we went public, Howard has been a straight and steady voice for Altair, and I wish him well in his next adventures, especially those involving his family and has worked with charitable organizations.
In choosing, our next CFO, it was important to me that we have someone with an operational focus and enthusiasm for doing the right things as we grow the company to the next level of revenue and profitability. Matt is clearly on target in this regard, given his financial expertise and large-scale software company experience. Matt will officially take over from Howard on March 16. And both of them are to be congratulated on their superb process of transitioning the role in a way that has truly benefited the Altair organization. I look forward to working with Matt as we remain focused on delivering great technology to our customers, while we scale our software revenues and increase EBITDA for sustainable long-term growth.
Now I will turn the call over to Howard and Matt to provide more details on our financial performance and our guidance for the first quarter and full year of 2021. Howard and Matt?
Howard N. Morof - CFO
Thanks, Jim. I appreciate the kind words, and I'm delighted to have contributed to the growth and success of Altair over the course of the past decade. I will cover our Q4 and full year 2020 results. While Matt will detail our expectations for 2021, as Jim mentioned, we delivered excellent fourth quarter results on the top and bottom lines, driven by software product revenue, well in excess of our expectations going into the quarter.
I would like to remind everyone that our seasonal billings patterns, coupled with the treatment of revenue under ASC 606 results in heightened seasonality in revenue and associated metrics with higher software product revenue recorded in our first and fourth quarters of any given year, and we expect this pattern to continue. We exceeded our revenue guidance for Q4, driven by strong growth in software product revenue and handily exceeded our adjusted EBITDA guidance, driven by the combination of strong software revenue performance and continued controls over our operating expenses.
We entered the quarter with a conservative perspective due to the uncertainties arising from COVID-19 since our Q4 typically includes a greater proportion of expansion and new revenue compared to other quarters. We are delighted to see that the strong software growth we generated in Q3 carried through to Q4. We previously noted that changes in certain currencies can have an impact on our revenue, expenses and cash flows, especially when those changes occur over relatively shorter time periods or when currency changes are more pronounced over time.
Accordingly, we believe it is meaningful to measure aspects of our performance on a constant currency basis. For 2020, currencies did have a positive impact of $3.8 million on billings and an insignificant impact on revenue and adjusted EBITDA for the year.
For Q4, currency positively impacted revenue by $3.2 million and had a nominally positive impact on adjusted EBITDA. Billings benefited in the quarter by $7.5 million due to currency moves compared to Q4 '19, primarily driven by moves in the euro.
Our fourth quarter results are attributed to continuing solid demand for our software products that exceeded our expectations. Software product revenue reached $113.6 million, an increase of 12.3% from a year ago and over 9% on a constant currency basis, while total revenue equaled $133.4 million, representing growth of almost 8% from the fourth quarter of 2019 and over 5% on a constant currency basis.
Acquisitions did not have a meaningful impact on revenue for the quarter. Total revenue continued to be impacted by the reduction in software-related and client engineering services revenue resulting from the continuing impact of COVID-19 on the demand for these services. Reflecting continued modest recovery compared to Q3, our software-related services revenue declined about 12% in the quarter relative to the prior year.
These results represent improvement compared to Q3, which declined 22% compared to the prior year. As expected, our client engineering services revenue declined by 15% in the quarter compared to the prior year due to reductions imposed by some of our CES customers, consistent with the prior quarter. Our 2020 results benefited from demand for our software products.
Software product revenue reached $391.7 million, an increase of almost 7% from a year ago, while total revenue equaled $469.9 million, representing growth of 2.4% from 2019, both exceeding guidance provided last quarter. Software-related services declined by 23.5% compared to a year ago, primarily impacted by the headwinds in our automotive customer base.
Our growth and investment strategies remain targeted and higher-margin software product revenue opportunities. Revenue mix continued its favorable trend in the fourth quarter. Software product revenue increased to 85.1% of total revenue. Up almost 350 basis points from 81.7% last year without any adjustment for currency or acquisition-related impacts, which were minimal.
This continues the important long-term trend of an increasing mix of software product revenue, a key driver to expansion of our operating margins. For the full year, software product revenue progressed to 83.4% of our total revenue from 79.9% for the prior year.
Our recurring software license rate that is the percentage of software product revenue that is recurring, continues to be strong with a healthy increase to 92% for the year as we continue to emphasize growth in our recurring revenue streams. Fourth quarter billings were $146 million, an increase of 12% from a year ago, driven by software momentum, indicative of the strong growth in our software product business.
For the year, billings were $480.4 million, an increase of about 1% from a year ago, driven by the growth in our software product business, offset in part by declines and software-related and client engineering services. We tend to view billings over longer time periods due to the impact variations in timing of renewals, expansions and new customer arrangements can have quarter-to-quarter.
I would like to shift to the balance of the P&L results. Gross margin in the fourth quarter improved to 75.5%, consistent with the revenue mix shift to software product revenue. On a full year basis, gross margin improved by over 300 basis points to 74.2% compared to prior year gross margin, driven by the positive shift in revenue mix.
For the quarter, non-GAAP operating expenses, which exclude stock-based compensation, amortization of intangible assets and other operating income were $81.9 million. The increase from $73 million in Q3 is consistent with our expectations of the impact of our acquisitions later in 2020 as well as restoring certain cost-reduction efforts undertaken earlier in the year.
Adjusted EBITDA for the quarter equaled $21.7 million, a terrific increase of over 70% from last year, driven predominantly by the increase in software product revenue, along with the benefit of reduced travel and selling expenses operating under COVID-19 restrictions.
In Q4, we did realize some revenue and incurred expenses related to the acquisitions of Univa, Ellexus and M-Base. The magnitude of these inclusions did not significantly impact our results, either on the top line or bottom line.
Adjusted EBITDA for the year grew over 45% to $57.3 million compared to adjusted EBITDA of $39.5 million a year ago, driven by healthy software momentum in the second half of the year, along with cost reductions over part of 2020 arising from COVID-19 actions we have spoken about previously. Our performance in 2020 pushed adjusted EBITDA margins in 2020 to 12.2%, up from 8.6% in 2019.
Turning to our balance sheet. Consistent with the typical seasonality in our billings and collections activities, we ended the fourth quarter with $241 million in cash and cash equivalents. We had $120 million in undrawn capacity on our U.S. revolver. Just note that in early January 2021, we repaid the $30 million we had drawn on the line of credit during 2020. We generated cash flow from operations for the year of $32.9 million compared to $31.4 million for 2019.
Our free cash flow improved to $26.8 million compared to $21.7 million for the prior year, due in part to reduction in cash expenditures for property and equipment. It has been a pleasure to have been a part of Altair in the past decade, working with Jim, the entire Altair team and more recently, the investment community.
Having spent much time with Matt during our transition, I am completely confident that the finance function will be in great hands going forward.
I would now like to turn the call over to Matt to discuss expectations for 2021. Matt?
Matthew Charles Brown - SVP
Thank you, Howard. I want to start by thanking Howard for providing a very thorough and seamless transition over these past 2 months. When I started on January 4, I could not have imagined a better handover of CFO responsibilities. He's left us in a great spot for the future. And he's assured me that if we need to call him, he'll still pick up the phone. So Howard, thank you. I very much appreciate it.
As you just heard, we're coming off a very successful fiscal year 2020 and especially Q4 in what was a challenging COVID-19 environment. We saw record software product revenue in Q4 and for the full year, with fiscal 2020 software product revenue growing 6.8% year-over-year.
Total revenue in 2020 grew 2.4% year-over-year. And while also a record high, we saw declines in our service revenue that partially offset the software product revenue growth. As some customers pulled back on their services and contract spend. As we look ahead to Q1 and fiscal year 2021, we expect to continue the momentum we saw at the end of 2020 and carry that into 2021 and are expecting software product revenue for Q1 21 in the range of $118 million to $120 million or year-over-year growth of 8.8% to 10.7%. And for fiscal year 2021, in the range of $423 million to $431 million or year-over-year growth of 8.0% to 10.0%.
In looking at total revenue for 2021, we believe our services revenue has stabilized. And although we do not expect to see service revenue declines in 2021, like we did in 2020, we are not expecting service revenue growth either as we continue into what is likely another mixed COVID-19 year.
As a result, we are forecasting total revenue for Q1 21 in the range of $138 million to $140 million or year-over-year growth of 5.0% to 6.5%. And for fiscal year 2021 in the range of $502 million to $510 million or year-over-year growth of 6.8% to 8.5%. We believe our revenue guidance is balanced. But will depend on the levels and speed of the post-COVID recovery, particularly in the later part of the year.
From a cost perspective, the company succeeded in limiting expenditures during 2020, while in the midst of COVID-19 uncertainty. In particular, during 2020, we reduced marketing and trade show costs by almost 40% relative to the prior year and reduced travel and entertainment costs by 68%. In 2021, we expect some of this activity to return to normal, resulting in an increase in costs year-over-year, particularly in Q2 through Q4 '21 as we anniversary some of those cost cuts and as travel resumes.
However, we will continue to be disciplined in our expenditures. We are currently looking at targeted reductions and employee costs, contractor costs and professional services spend as we reorganize within the business. We expect these reductions to be substantially complete in the first half of 2021, freeing up capacity for investments in our product technology and sales capacity.
As a result, we expect to incur cash reorganization costs in the first half of between $5 million to $7 million, which is excluded from our adjusted EBITDA. For Q1 '21, we expect adjusted EBITDA in the range of $24 million to $26 million or year-over-year growth of 10.7% to 20.0%, which translates to adjusted EBITDA margin of 17.4% to 18.6%.
And for fiscal year 2021, we expect adjusted EBITDA in the range of $58 million to $66 million or year-over-year growth of 1.2% to 15.2%, which translates to adjusted EBITDA margin of 11.6% to 12.9%. Our expectations on adjusted EBITDA are inclusive of the cost reductions and investments I mentioned a moment ago.
We expect free cash flow for the year to increase in line with the increase in adjusted EBITDA and will be impacted by the reduction related cash costs and are forecasting free cash flow for fiscal year 2021 in the range of $26 million to $34 million. As a reminder, our cash flow expectations are sensitive to billings and collection patterns, which fluctuate seasonally.
We've provided detailed guidance tables in our earnings press release, which was issued after close of market yesterday and includes guidance on non-GAAP net income. Moving forward, when measuring non-GAAP net income, we will exclude noncash interest expense and will apply a consistent non-GAAP income tax rate. We've made these changes to reflect management's view of the business and to be more consistent with our peers. These changes will be applied to comparable prior periods to aid in comparability and have been provided in the supplemental tables in our earnings press release.
COVID-19 developments continue to evolve, but we are cautiously optimistic on a broad economic recovery as we look ahead to 2021. Altair has never been better positioned to support our customers on solving some of the most challenging problems faced by engineers, scientists and data analysts.
Our simulation products are being increasingly adopted earlier in the design process, improving product performance and reducing costs for our customers. Our HPC products are enabling customers to maximize their compute resources and our data science and preparation tools are leveraging AI to enable customers to organize and visualize data to make important decisions quickly. We see these solutions converging and providing real value to our customers.
With that, we'd be happy to take your questions. Operator?
Operator
(Operator Instructions) Our first question comes from Rich Valera with Needham.
Richard Frank Valera - Senior Analyst
Congratulations on a strong finish to the year. And Howard, best wishes on your next chapter and welcome, Matt. And with that, I wanted to just talk about how the quarter played out versus your expectations, really nice upside in the quarter. And I'm wondering, was that a result of things coming in, have you actually seen improvements during the quarter or just conservatism yet embedded into the guidance because you were kind of concerned about a COVID second wave, which I think you had mentioned on last quarter's call. Just wondering where that upside came from on the quarter relative to your expectations.
James R. Scapa - Founder, Chairman & CEO
Rich, we're on different places.
Howard N. Morof - CFO
Jim, go ahead or I'm happy to answer.
James R. Scapa - Founder, Chairman & CEO
Yes. Why don't you answer that, Howard? That would be great.
Howard N. Morof - CFO
Sure. Sure. So Rich, and thank you for the nice words. We certainly entered with a sense of conservatism as we've frankly been pretty cautious over the course of the entire year for obvious reasons -- for most of 2020 for obvious reasons, so we entered the quarter conservative. We know that in Q4, we have a little higher proportion of new and expansion revenue as is typical in our Q4. So the conservatism that we thought was very well warranted. Against that backdrop, the quarter continued to, I would say, improve as the quarter navigated along reflecting growth investment, strength in our customer base. And frankly, continued use in growth and adoption of technologies that are ever so critical to our customers. So I think it was really sort of a combination of conservatism and as well, new expansion in the way that you would hope to see.
Richard Frank Valera - Senior Analyst
That's great. And then maybe for you, Jim. Great to hear about your success selling kind of converged analytics and simulation solutions. But just wanted to get your sense of where you are in that path towards creating that converged model kind of relative to maybe where you thought you were going to be when you made the acquisition and where you think it could be say, 5 years from now, what you've done so far? And maybe what you're doing to further that integration? And then just where you are in sort of the model transition for that analytics business?
James R. Scapa - Founder, Chairman & CEO
Thanks, Rich. Nice to hear your voice. So I mean, I think that the model transition is largely done at this point. We've really moved things over to subscription pretty well at this point, and we're kind of on par with the rest of the business into this year. But that part is done. As far as sort of this convergence between the engineering and the data analytics, I think it's a very natural transformation that's really happening. It's happening within our products and in ways that customers and users don't even know it's happening to some extent. And it's also happening for customers who really are beginning to recognize the opportunity. So we have almost every one of our account managers has opportunities at this point that are really taking advantage of this convergence. And it's really starting to engage. We're starting to understand use cases that makes sense as we have success and point those use cases to other customers. So 5 years from now, I don't think we're going to be talking about sort of a difference between simulation and AI. I think it's all going to be computational science basically that we're talking about.
Operator
Our next question comes from Jackson Ader with JPMorgan.
Jackson Edmund Ader - Analyst
I'll echo Rich thoughts, Howard, it's been a lot of fun and welcome, Matt. First question on the reorganization. What areas of the sales force or I guess, what types of sales investments will you guys be making? Where are you going to be shifting chips away from and being placing bets?
Matthew Charles Brown - SVP
Thanks for the question. Jim, do you want to take a stab at that and pass it to me?
James R. Scapa - Founder, Chairman & CEO
Yes, I think I should answer that question. Sorry, Matt. So in terms of where we're continuing to invest, we are continuing to invest in enterprise-level customers who we see large opportunities with. So we're beginning to target our direct account knowledge more and more at those opportunities and will create more focus for them. We're creating swim lanes really in the market and working the direct account managers of the accounts that make sense for these enterprise opportunities. We have inside sales and business development, working more down market and indirect also down market in combination with them. So we're continuing to make investments. We're just -- we think we're just getting smarter at how we're organizing and leveraging the sales resources and the capacity that we have.
Matt, can you answer something there? I'm not sure what he's going to...
Matthew Charles Brown - SVP
Yes. I mean I would just chime in and say, from a high-level context, right, I mean, this is over the past 5 years, we've done 23 or so acquisitions and continue to really refine our operating model. And so that's how I would characterize this. We're going to continue to invest in our product technology and in our sales engine moving forward. And so this is really a refocusing is the way that I would characterize it.
Jackson Edmund Ader - Analyst
Okay. And then a follow-up, I guess, directly for Matt. What's the FX tailwind in 2021 for the revenue growth guidance?
Matthew Charles Brown - SVP
It's a couple of points. I think we are -- obviously, as we guide '21, we're using today's rates on the FX. So you're not -- we're not anticipating movements in the FX rates. But within the -- comparable to the sort of year-over-year impact, it's worth a couple of points of FX on the revenue line, but next to nothing on EBITDA. Those naturally offset.
Operator
Our next question comes from Bhavan Suri with William Blair.
Bhavanmit Singh Suri - Partner & Co-Group Head of Technology, Media and Communications
And I'll echo my congrats on the quarter. Welcome, Matt. And Howard, we will miss you. I'm sure we're running each other again, but best of luck. I guess I want to touch on a couple of quick things. One, maybe this one is for Jim. Jim, the services business naturally, obviously, this year or in 2020, had challenges just COVID, et cetera. But as you think about it, one of the great things about that business was you got really intimate with customers. Like you got to understand what they were doing, what they wanted, it drove some product development or at least ideas around innovation. And as you think about '21 and '22, do you think you reinvest in that business a little bit to sort of keep that customer intimacy, to keep those guys at the customers to help them with their solutions, but also to help guide and drive product innovation. How should we think about that playing out?
James R. Scapa - Founder, Chairman & CEO
We are still very, very actively engaged with a lot of customers on, I'll say, very advanced projects in electric motors, batteries, those kinds of things, additive manufacturing, simulation, all of that. The service business that declined, if you will, last year, is closer to -- it's a continuum of super-advanced stuff that we do down to more commodity level. And we don't do the very commodity anymore over the last 5 or 10 years. But still that are being lost, if you will, or more of the things on the commodity level side. So we're still very engaged with customers. Customers still recognize the technical superiority of Altair. And it's advancing our products still, but it's also advancing these relationships.
Bhavanmit Singh Suri - Partner & Co-Group Head of Technology, Media and Communications
Got it. And then maybe one for Howard and Matt combined maybe, but you -- Howard had a philosophy over the x number of years about guidance with Jim and team. And I guess, Matt, as you look at the guidance for 2021, given there is still risk in COVID, and you think about sort of billings was up 1% or a little above that in constant currency. I guess what are you seeing in pipeline, or are customers in the auto space hiring more engineers, are you seeing close rates improve, lands improve, the size of the initial land improve, expand, improve, that sort of gives you that visibility confidence. Again, just the business itself is not choppy, but the customer adoption, customer buying behavior can be choppy. So just trying to understand how you guys have thought about the guidance and sort of what you bring to the table in terms of visibility. I'd love to sort of think about that, both tactically for 2021 and the strategic, how you think about guidance?
Matthew Charles Brown - SVP
I appreciate the question. So I would say that we are cautiously optimistic about 2021. And you can kind of separate top line at least into the 2 sections, one being software product revenue and the other being all else, right? So if we start with all else, we're still in a mixed COVID year. As you know, last year, Q1, not really impacted by COVID. This year, Q1 growth. And as we move forward throughout the rest of the year, we're expecting some bit of recovery. But net-net, our expectations on everything other than software product revenue is that the year is going to be basically flat to 2020. And so then that leads you with software product revenue, and we're pretty optimistic there. We're seeing good engagement from our customers. We're seeing a healthy pipeline. We feel very strongly about our technology and how we compete. And so that's where you're seeing the growth. And of course, we'll have to see how the year plays out. But like I said, we're cautiously optimistic there.
James R. Scapa - Founder, Chairman & CEO
If I could add to that. I think sometimes it's undervalued by you guys, but our recurring revenue is, I think, in 2020, it was something over 92%. So we go into the year and very, very sturdy even in 2020. So we go into the year with a pretty high recurring revenue. We expect the attrition on that to be a little bit better than in the prior year. And when we look at new and expansion, of course, we were a little bit weaker last year there. But we can judge how weak we were against where we expect we might be. And all of that together gives us that cautious optimism as well.
Operator
Our next question comes from Ken Wong with Guggenheim Securities.
Hoi-Fung Wong - Senior Analyst
This one probably for you, Jim. In the data space, you have a southern California-based competitor that saw a little bit of softness. And just based on what we're hearing from you guys, it looks like you feel that your business has powered forward quite nicely. Just wondering maybe what some of the trends you're seeing there and maybe what some of the differences are in terms of why you guys seem to be surging forward where they seem to be seeing a little bit of weakness?
James R. Scapa - Founder, Chairman & CEO
Sorry, I can't speak to the other guys. But for us, I think we have very, very strong solution on the data prep side that just continues to go forward. But I think the rest of our offering, frankly speaking, is quite a bit deeper and broader than the guys, I assume, you're alluding to. And frankly, all of that integrated with the rest of our business and how we're targeting things gives us this optimism.
Hoi-Fung Wong - Senior Analyst
Got it. And then just one for the Matt and Howard combo. I guess, on the EBITDA guide, it doesn't look like you guys are getting much leverage next year on a full year basis, it looks like it's going to be flat from a margin perspective. Can you maybe just walk us through some of the moving pieces on the spend side, whether it's reinvestment versus a rebound in discretionary or anything else we might be missing from a spend perspective that maybe keeps us from getting more leverage on the -- on EBITDA?
Matthew Charles Brown - SVP
Sure, Ken. I'll take that. So you may be underestimating that a little bit. I think if you look at how we exited Q4 from a spend perspective, that really is probably your best basis as you move forward through the year. And so you can see we carried that into the Q1 guide. In terms of the spend, it's not much incremental spend from Q4 to Q1 at the midpoint of the guide. But then as you look forward through the year, you actually will see off of that run rate. You will see a reduction in our expected spend that's implicit in the guide. So make sure that you're taking that into account. There's some nice efficiencies there that are coupled with investments in our technology and in sales and some headwinds that we're seeing from some return to normal on travel and marketing expenses. So I think it's a pretty -- I think it's a pretty fair guide and actually, I'm pretty happy with that outcome.
Operator
Our next question comes from Brian Essex with Goldman Sachs.
Brian Lee Essex - Equity Analyst
Maybe, Jim, a question for you. In your prepared remarks, you gave several nice examples of customers significantly restructuring their relationships. But where -- what are the overall trends with regard to adoption of product roots within the Altair unit platform? Are you seeing a meaningful number of enterprises buy for greater flexibility maybe, for example, buying multiphysics instead of mechanical engineering to enable usage of perhaps SimLab and Inspire and other adjacent applications.
James R. Scapa - Founder, Chairman & CEO
So the answer is yes. Actually, the ultra-units model has been very, very well received. It's well received by my account management team. They really appreciate it because it gives them the ability to get the right value and customers that are willing to pay more for the extra service features in our products. And it lets us basically be competitive in gaining markets, where the customers are more price sensitive. So I think our guys are seeing it well. I think the customers are appreciating it as well, actually, because it's a very, very fair model for them. And we are seeing adoption for sure.
Brian Lee Essex - Equity Analyst
That's good to hear. And maybe just a follow-up to Bhavan's question. On services, is the bench of services talent that you have access to limited to a certain group of businesses or verticals or perhaps certain technology versus some emerging technology? In other words, how applicable are those services as you continue to extend your model into emerging businesses or additional incremental technology through acquisitions?
James R. Scapa - Founder, Chairman & CEO
We have very, very significantly shifted bill sense that we have -- I mean, if you look at the projects that we're doing in electronics, electric vehicle, electric mode sensors, IoT devices, we're at the state-of-the-art, frankly, battery design. I think you'd be surprised at the expertise that we have now.
Operator
Our next question comes from Matt Hedberg with RBC Capital Markets.
Matthew George Hedberg - Analyst
Congrats from me as well. And obviously, Howard, great working with you, and Matt look forward to working with you again. I just had 1 question, and it's really a follow-up to Brian's question on Altair units. Obviously, you're seeing success there, and it opens up new opportunities. I guess the question is, when you think about the 2021 guide, how do you contemplate how pervasive, or maybe said differently, the impact on top line growth with increasing usage of Altair units. Just wondering how you sort of conceptually thought about that as you contemplated the outlook for '21?
Matthew Charles Brown - SVP
Yes. So...
James R. Scapa - Founder, Chairman & CEO
Matt, I'll answer that, but I personally I'm not thinking about it at that sort of granular level. I'm trying to model that. I don't know if Matt or Howard have been trying to do that. For us, we're strictly looking at what we see the pipeline looking like.
Matthew Charles Brown - SVP
I can chime in there, too, Jim. So Matt, good to hear from you. So some helpful context, I think. So far, as of now, roughly 1/3 of our customers have actually already converted. So I think that's important to note. And we're going to expect the rest of those customers to be substantially completed within the next year. So we'll see that throughout the year. The way that I'm thinking about it is that as these customers convert, they're finding their way into the suite that makes the most sense for them. In some cases, it ends up being a slight price increase, in other instances, they're finding a suite that makes sense for them where they can utilize the -- exactly the products that they want. And so in the first year, I'm not expecting a meaningful impact. But what I think it does is it sets us up for the future in a way that allows these users to expand within the suites that they're really using, and it allows us to get the value for that, particularly at the enterprise level. So hopefully, that answers your question. We're not baking in a meaningful impact in the year. And so far, we've seen 1/3 of our customers already converted.
Operator
Our next question comes from Gal Munda with Berenberg Capital.
Gal Munda - Analyst
The first one is just a little bit in the past, you've talked about how the usage has trended, especially towards the end of the year, kind of give us a little bit of an update over the year. And maybe I'm not specifically asking about the quarter itself. But just in general, how have you seen usage on solvers evolving throughout the 2020? That would be very helpful.
James R. Scapa - Founder, Chairman & CEO
Gal, first of all, good morning to you. I think your question was how do we see the usage of solvers evolving? Is that correct?
Gal Munda - Analyst
No. How has evolved and how did you see it? How -- what was kind of the performance in 2020? Just as an overall, considering the fact that (inaudible)?
James R. Scapa - Founder, Chairman & CEO
Sorry. The solver suite that we have continues to gain share, we think, and grow. Almost in every area for us actually. So if you went back 10 years ago, we were primarily a pre-post company today. A very significant part of our business is really on the physics side of the business, and that's going to continue to grow even more as time goes on. So solvers are an ever-increasing part of our overall business and strategy.
Gal Munda - Analyst
Okay. That's helpful. And then in light of your comments about kind of being a little bit more mindful of the way you invest in sales and really trying to attack those enterprise-level sales from -- and rebasing some of the other efforts, either into the indirect channel or build out. How far would you say you've progressed on the indirect sales channel? Now you've talked in the past about your ambitions to significantly increase contribution to revenue from that side. Was 2020 kind of a year of foundation and you expect that to come through in '21? Any color on that?
James R. Scapa - Founder, Chairman & CEO
Sure. We've made some progress, I would say, and it depends on the geography. But from my point of view, not as much progress as I would like to see. Particularly in the Americas. So we -- I think the inside sales, the business development piece of the business has really gotten a lot of traction for us. I think the indirect is getting traction more and more in Europe and continuing APAC. And in the Americas, I think we have some more work to do, quite frankly.
Operator
Our next question is from Mark Schappel from Benchmark.
Mark William Schappel - Director of Research & Equity Research Analyst
First off, Howard, the best to you going forward. It was good working with you. But Jim, a question for you. I was wondering if we could dig a little bit deeper into your strategic partnership with GE as part of the acquisition. I was wondering if you just could provide some additional details on the arrangement and what we should expect from it going forward?
James R. Scapa - Founder, Chairman & CEO
So I mean this is a long coming partnership for us. We've been working with GE for a couple of years now. With this technology, it was actually part of the Altair Partner Alliance. And now the spin-out from GE to be something that we've taken over responsibility for 100%. There's a very, very large user base of this technology inside of GE. It's in the thousands. And they're excited, I think, about having a commercial software company take responsibility for it, and there's a lot of opportunities to leverage this to grow the partnership in many different directions with GE around all of our software. So that part, we're very excited about. There's other projects that we actually have been doing with GE in the area of rotating machinery and others, independent flow simulator project.
Coming to flow simulator, it's a really terrific piece of technology that was developed inside of GE. Very often, if I can be candid, the OEMs have a lot of internal development that they do. But very often, it's not all commercial grade. And they often are looking to spin it out and very often, it's not really competitive. This is not the case here. This is a great piece of technology. It's getting traction. A lot of customers for us. And we're very, very excited to have it with us. So it's just a very nice partnership with GE. It's a great piece of technology, and it's a model. For things that we think we can do with some other customers as well.
Mark William Schappel - Director of Research & Equity Research Analyst
Great. Very helpful. And then an additional question here. A quarter or 2 ago, you released your Inspire Mold solution. I was wondering, and that was a pretty significant capability for the plastics industry. And I know it's still early, but I was wondering if you've had any Inspire Mold wins at the end of the year here that you can talk about?
James R. Scapa - Founder, Chairman & CEO
We have some loans, but probably nothing that I can reach into my bag of tricks and speak about in a coherent way. It's pretty early days, and we see a lot of interest in the product. And we're pretty hopeful that we're going to see a lot of traction around it, but it's frankly a little early. Ask me in another quarter or 2, and I'll tell you how we're doing. I'll make sure to study it more before the next call.
Operator
And this concludes our Q&A session for today. I would like to turn the call back to our CEO, James Scapa, for his final remarks.
James R. Scapa - Founder, Chairman & CEO
Thank you. So in conclusion here, I just want to thank everyone for their support and feeling great about 2020 with so many challenges and very excited about 2021. And once again, just want to say a final thank you to Howard for just being a wonderful partner for me for so many years, initially on the Board and then coming as CFO for the company, helping us to transform our finance department, helping us to go public and just be a wonderful partner for me for so many years. So wishing Howard the best, and thanks to everyone else. See you all.
Operator
Ladies and gentlemen, thank you for your participation in today's program. You may now disconnect. Have a wonderful day.