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Duolingo filed to go public yesterday, giving the world a deep look inside its business results and how the pandemic impacted the edtech unicorn’s performance. TC’s initial read of the company’s filing was generally positive, noting that its growth was impressive and its losses modest; Duolingo recently began making money on an adjusted basis.

While the company’s top-level numbers are impressive, we want to go one level deeper to grow our understanding of the company beyond our EC-1.

First, we’ll explore the growth of Duolingo’s total user base, how much money it makes per active user, and how effectively the company has managed to convert free users to paid products over time. The numbers will set us up to understand what else can be learned about Duolingo’s business beyond our original deep dive into the company’s finances — specifically underscoring the pressure cooker it finds itself in when looking for new revenue sources.

Starting with Duolingo’s growth in total active users, guess how fast they rose from 2019 to 2020. Hold that number in your head.

The actual numbers are as follows: In 2019, Duolingo closed the year with 27.3 million monthly active users (MAUs); it wrapped 2020 with 36.7 million MAUs. That’s a gain of 34%. If we narrow our gaze to Q1 2021 numbers compared to Q1 2020, we can see that Duolingo’s MAUs rose from 33.5 million to 39.9 million, or growth of around 19%.

The bulk of Duolingo’s growth, then, came in early 2020 when we consider its pandemic bump. Put more simply, the company scaled from 27.3 million MAUs at the end of 2019 to 33.5 million MAUs at the end of Q1 2020; from then, the company added 3.2 million more MAUs throughout 2020 and 6.4 million during the next four quarters.

Another lens through which to view the numbers is simply a recognition that first-quarter results at Duolingo appear to be stronger than results in the rest of the year, perhaps due to New Year’s resolutions to learn a new language or brush up on a second language learned in high school.

Next, let’s examine Duolingo’s monetization efforts regarding converting free users to paying users.

Here we can see a very different growth story. While the company’s MAUs rose 34% from 2019 to 2020, the company’s paying users rose from 900,000 at the end of 2019 to 1.6 million at the end of 2020. That is a far-sharper gain of 84% on a year-over-year basis.

So, while Duolingo did see material user growth during 2020, it saw turbo-charged expansion in the users it was able to shake revenue from. Improved monetization, more than acceleration in user growth, was the pandemic’s effect on Duolingo.

What can we see in the company’s more recent results? From Q1 2020 to Q1 2021, Duolingo’s paid subscribers rose from 1.1 million to 1.8 million, a gain of around 64%. That was a slower pace than the company managed more generally in 2020, which matches Duolingo’s slower revenue growth in Q1 2021 than it recorded in 2020.

The number is still strong, we think. But not as impressive as the more than 100% revenue expansion that the company put on the board last year.

In percentage terms, 3.3% of Duolingo’s MAUs were paid subscribers in 2019. That figure rose to 4.4% in 2020. And in Q1 2021, it reached 4.5%. Duolingo rounds that number to 5% in its S-1, which feels somewhat aggressive to us, given the somewhat modest pace at which the metric is improving. Here’s the wording:

As of March 31, 2021, approximately 5% of our monthly active users were paid subscribers of Duolingo Plus. Our paid subscriber penetration has increased steadily since we launched Duolingo Plus in 2017 and, combined with our user growth, has led to our revenue more than doubling every year since.

A gain of 0.1 percentage point in a quarter is growth, we suppose.

Next, let’s chat about revenue per MAU. To get consistent numbers, we’ll divide quarterly revenues by MAU figures from the same period. So, we’ll compare Q4 2019 revenue at Duolingo with its year-end MAU figure. We’ll do the same for 2020, and for Q1 2021 we’ll use both numbers from that period.