Apple Music: also on Android. Photo by tualamac on Flickr.

Apple announced on Wednesday that it now has 20 million subscribers to Apple Music after just 18 months – which feels like pretty good progress. Apple Music is also the only meaningful Apple service that’s also available on Android, as Apple tries to spread itself cross-platform.

Why is Music on Android as well as iOS? Because it’s not a distinguishing feature. Unlike Apple’s iMessage or its App Store, both of which are exclusive to Apple, and whose features are unique to it, you can already get lots of music services on both iOS and Android – Spotify, Tidal, Deezer, Google Play Music, Amazon Music and so on. (Not many, if any, of them are making money, but leave that aside.)

For Apple, every additional subscriber to Music is a bonus; it’s all money. The ones on Android are potential converts to iOS, where there’s more money to be made selling them iPhones, iPads and Macs, but time spent on Music is time not spent on Spotify, its principal rival, so that’s a benefit to Apple. App development costs aside, which are comparatively small, Android is a benefit. Additionally, if someone with an iPhone gets a Family Plan (allowing five people to use the same account; a Family Plan only counts as one subscription in Apple’s total), those five could include Android users.

So how many of those 20m are on Android? I’ve been tracking the stats on Google’s Play store for Apple Music downloads since its launch, including the download range and the number of 1-, 2-, 3-, 4- and 5-star reviews.

There are two things we need to work out: how many downloads there have been, and how many of those have resulted in subscriptions.

Download power

To estimate the number of downloads, I track the definite waypoints – when it passed 5,000, 50,000, 500,000, 1m, 5m – and the number of reviews against them.

The process is very standard across all apps: to begin with comments come quickly, so that almost every download prompts someone to review (about every 27 downloads, someone left a comment up to the 500,000 mark) which then tails off (only one in every 100 downloads prompted a comment by the 10m download mark). You can thus model how the number will change; and so even when you’re in the vague space between waypoints, you can estimate the number of downloads. (I’ve used this technique to calculate Android Wear downloads; it was accurate about when it would hit the 5m mark by a few weeks in a two-year timeframe.) The R-squared value is the correlation, which runs between 0 and 1: closer to 1 is nearer exact correlation.

Based on that, I calculate that this week Apple Music on Android hit 12.75m downloads. (It passed 10m around September 6.) It’s adding about 230,000 downloads per week, at a rate that seems to be holding pretty steady.

But downloads aren’t subscriptions. Apple Music offers a three-month trial period, after which you have to pony up. Clearly, some people will drop out. But what proportion of those who download it stay on and subscribe?

One subtlety here: the three-month trial means that strictly, we should ignore the downloads made in the past three months, since none of them will have qualified to become subscribers yet. (Downloads made to join a Family Plan don’t count as subscriptions.)

So the figure we’re looking at is from 6 September – which just happens to be when Apple Music hit the 10m download mark.

Star quality

Here’s an obvious way to estimate that: look at the high-quality reviews. It seems logical (Captain) that five-star reviews indicate people who are really happy with the service. Four-star reviews are people who are pretty happy, but find some hassles with it.

By the time you’re down to three-star and below, I think you’re talking about people who aren’t impressed and won’t be staying.

Although people don’t become subscribers until three months have elapsed, I think you can include recent ratings, since those could come from people who have become subscribers. (We don’t know what prompts people to review an app.)

So what do the ratings show? At present, five-star reviews are about 44% of all reviews; four-star ones, 11%. That ratio has been pretty consistent; five-star reviews have been at least 38% of all, and average 42% over the life of the app. Four-star reviews go down to 10%, and average 11%. (I don’t know what the average is across all apps on Google Play.) In fact, the data shows a gradual improvement in how the app is perceived, according to the reviews.

Based on this information, we can get some useful minima and maxima.

10m downloads, five-star users subscribe: 4.4m Apple Music subscribers on Android

10m downloads, four- and five-star users subscribe: 5.5m subscribers

12.75m downloads, five-star users subscribe: 5.61m subscribers

12.75m downloads, four- and five-star users subscribe: 7m subscribers.

I’d suggest the useful range is probably the 4.4m-5.5m one.

(One confounding caveat: we don’t know how many of the eager reviews come from people who downloaded it because they’re part of a Family Plan. I can’t think of a simple way to evaluate this unknown.)

Phoning home

If we accept those numbers, it suggests there are 14.5m-15.6m Apple Music subscribers on iOS.

What does that mean in the context of how many phones are out there?

There are about 550m iPhones in use, according to Neil Cybart. And there are around 1.2bn Android phones in use. (Apple Music is available in China, so it can run on phones there.)

This implies quite low penetration for Apple Music: about 2.5% on iPhones, and about 0.3%-0.4% on Android.

Then again, given that Spotify’s last published figure (in September) is 40m subscribers, and it is also available on both platforms, it’s clear that it’s just difficult to get people to sign up to these services. Given how many people Spotify has been available to in multiple countries, it has only been able to convert about 1% of the total available internet population during its life. It seems like getting people to sign up to music streaming really isn’t easy at all. So if you’re an Apple Music or Spotify subscriber, you’re very much in the minority.

Even though Apple’s progress in a short time looks strong compared to Spotify’s over the same period, it’s an open question how big the total addressable market is here. Are we just crossing over from the early adopters to the broader audience who will jump on streaming? Or is it going to struggle to break through? These are still open questions.