THE ENGINE - a framework for the growth of mobile applications

A popular set of metrics against which the growth of applications can be measured is the pirate metrics or AARRR metrics:

If you interpret the framework literally, it leads to the following diagram:

New user --> Activated User --> Retained User --> Referral --> Paying User

This framework is useful in that it appropriately describes different steps in the lifecyle of your user on which you should be able to work on separately. However, like every framework–including the one presented here–it has its shortcomings. Mainly:

  1. The model does not necessarily reflect the lifecycle as you design it in your application, the order of the different steps and how they relate to each other. For instance, you might monetize users way before you can call them retained or encourage them to refer other people.
  2. The framework is rigid, invites you to think in a linear fashion2, when customer applications are all about loops. As a result it is not always clear what you need to optimize first in your application, where your biggest leverage is; improving one step of the lifecycle may actually improve every other step.

When working on growth, it is important that you have a clear view of the current user lifecyle in your application. To be able to find the biggest levers to pull, you need to graduate from this linear representation.

The Engine

When I worked on growth at my previous gig, I liked to focus instead on what I called the "engine": the set of loops that have the potential to make the growth of your application self-sustainable. Without an engine you are relying solely on external supply, which will eventually die out because of churn.

You can think of the framework as a 2-dimensional version of AARRR. In the following, I will mostly focus on acquisition and referral.

Free applications: the importance of viral loops

Acquisition can come from 3 channels:

  • Paid acquisition
  • External (linear) acquisition
  • Viral acquisition (word of mouth, invitations, etc.)

External acquisition may be thanks to your website, a post on Hacker News. It brings in a constant number of users, but does not scale. We include paid acquisition as an external channel for free applications: it only increases with the amount of money you have in the bank, not the number of users that you have.

Viral acquisition on the other hand is a result of having new users who get activated and remain long enough to refer their friends, who become new users, etc. Viral acquisition forms loops.

We can redraw the AARRR diagram for our free application as follows:

External --> New user --> Activated user --> Retained User
                |                                |
                |                                |
              Referral <-------------------------+

Your engine is viral acquisition. It is the only viable channel in this situation, the only one that loops. The one you should concentrate your efforts, but the hardest to work on. I suspect many startup spend a lot of money in ads, because it is much easier than spending time improving their engine. It often doesn't pay off.

There may be different viral acquisition channels in your application, and they may occur at different times in the users' lifecycle. For instance, at my previous company, all of our initial growth was driven by a watermark on videos that people shared in Instagram. Although it is a very cheap loop, it allowed us to scale to the hundreds of thousands of active user without any acquisition effort. A second viral loop was inviting users to share their music directly to friends via messages, Instagram direct messages, WhatsApp, Messenger… a pretty strong loop here too.

While the arrow I drew to Referral started at Retained User, this does not necessarily have to be in the case. We decided to move up the music sharing up on the onboarding (this is what most users came in for) so more users could 'work' on the viral loop. The engine roughly looked like this:

                |              |
External --> New user ----Onboarding----> Activated user --> Retained User
                |                                |               |
                |                                |               |
              Referral <-------------------------+---------------+

This was a huge success, and gaining the 40 percentage points or so by starting the loop from the onboarding led to a further increase in active users. You can see that thinking in terms of loops is way more flexible. It can map the lifecyle of users in your application more accurately.

Growth is about, first and foremost, crafting loops in your application to build a reliable engine. Without an engine you don't have a rocket, but a leaky bucket.

Not all loops are created equal

There are usually several levers you can pull to improve the engine. If I take the example of the watermark loop mentionned earlier, we could work on two aspects:

  1. Improve the conversion song shared --> new user. There are several things we could work on: how visible and appealing the watermark was, the hashtags that we inserted by default in the post, appealing to the users that have the largest number of followers, etc.
  2. Improve the multiplier by increasing the average number of track shared per user.

Assuming that the conversion rate \(r\) is not affected by the number of shares \(N\) per user (not true in practice), the average number of new users coming from this loop is given by \(r * N\). This looks ridiculously obvious, but now that you formalized this, it becomes easier to quantitatively compare your options thus make decisions. At the time, it seemed easier for us to work on the average number of tracks shared by user, so we made it a priority, and it served us well.

It is therefore important to map your viral loop and write down the conversion rate between each steps and the multipliers. Take your app, a whiteboard, and go through it carefully. Pull your analytics and write down the numbers. It will often be obvious which levers you should pull once that work is done. Don't forget to write down timescales sa well: an ok performing loop activated daily is always better than an incredible loop that is activated once a week. Growth hacking is all about building and tuning your engine so your application's growth self-sustains.

If you don't map your engine then you're just navigating in the dark, waiting for the lucky strike. But luck should have nothing to do with this. Looking for growth should be tactical.

Several channel will work, at different stages in the life of your application. Some will be sustainable, some will not, there are a million possible solutions to improve it. So monitor the life of your engine closely. It is usually easy to write down a formula that sums it up in one number (e.g. the average number of users that come in the application after a referral per day per user in the application, or time-adjusted K factor), and follow it on a dashboard. You only need to look at the loop-level details when there is an issue.

Having said that, don't get too lost in the details of your map. Always be looking for the 10x improvement. You'll have time to worry about the extra 10% once your MAU hits the 100,000,000 bar.

Monetized application & paid acquisition loops

Paid acquisition gets a bad rap these days: they kill startups, they don't scale, etc. This is true if your application does not bring any money in the bank, and if it has no viral loop to kickstart. However, if you have some sort of subscription plans, paid acquisition can be transformed into a growth loop as well. The idea is simple:

Paid Acquisition ----> New User ---> Revenue
     |                                  |

For this to work and be sustainable, you need to make sure that the revenue you make per new user is way above the acquisition cost. Then you have a money printer, and you can scale your paid acquisition, acquiring more and more as people get in the funnel. Along with viral loops, this can be a killer combination.

Here is what our engine looked like with paid acquisition:

                +-------------------------------------------- Paying User
                |                                                |
External --> New user ----Onboarding----> Activated user --> Retained User
                |              |                 |               |
                |              |                 |               |
              Referral <-------------------------+---------------+

Word of caution: do not compute user revenue in terms of Lifetime Value, unless you have an unlimited amount of cash and all the time in the world. Timescales matter when it comes to paid acquisition, and you should think hard about them. If it takes you 5 months to repay the acquisition cost of one customer, and you do not have much cash to spend, you are buying yourself a very slow growth loop.

We made that mistake and realized it too late. We had a very good LTV on new users as retention on our paid plans was extremely good and the prices, well, high. But it took several months to repay the acquisition of one customer. The cash we gave to Facebook would have probably been better spent on someone working full time on other loops.

So don't think in terms of LTV in early-stage, it is more useful to think in terms of

  • Growth loop: is this acquisition channel sustainable? Does it bring me more cash than I spend? If this is your only growth loop, it better repay itself.
  • Timescale: How long does it take to repay itself? A day, a week? (worth doing) A month? (think hard) 6 months? (don't do it).

In summary:

Paid acquisition is interesting if it pays back quickly, or to kickstart an organic growth engine.

Other lessons

To finish, here are a couple of things I that learned working on growth:

  • Improving the onboarding is always a huge leverage. The further you go down in the onboarding funnel, the fewer users are still with you. And it can drop quickly. Engineering a loop very early in the funnel is always a very good idea.
  • Think about where you can position your loop as much as how your loop is implemented. I've seen too many referral schemes hidden in the application.
  • When you A/B test a specific part of the engine, you need to be considering the whole engine as well as the part-specific metrics. A growth engine is a system, not just the sum of different parts. Crafting a loop in your onboarding may increase the number of referrals, but you may have shot yourself in the foot by killing retention. Think in systems.
  • K-factor is meaningless. Time-to-referral (TTR) can be more important. A K-factor of .3 with an average one day TTR will blow any app with a K-factor of 1.1 with 7 day TTR out of the water. A better measure is K/day.
  • I'll say it for the 5th time because I don't see it often in discussions: timescales matter more than anything else when it comes to growth. Time is insanely valuable in early stage.



I do not think that the framework was meant to be interpreted linearly, but I have seen many people do so. It's worth repeating.


I don't think it is a useful metrics in itself, usually there is a reason why you want user to be retained: more revenue, more referrals, more DAU?