It should come as no surprise to learn that in the short time it’s been available, Fortnite: Battle Royale’s mobile version has been a massive success. In less than a month since it launched, the game has earned over $15 million from in-app purchases, according to market researcher Sensor Tower.

Following its invite-only beta release on iOS last month, Epic Games opened Fortnite up to all users of Apple’s mobile platform on Monday. On that day, players spent a massive $1.8 million, around three times the $620,000 daily average they spent on previous days.

Thanks to its 11 million global downloads, the free-to-play title now sits at number one in the Apple app charts. The game’s daily revenue had been gradually increasing in the days before the invite-only period ended, jumping to a record high on Monday. In the past seven days, players have spent $6.4 million on Fornite, putting it ahead of money-making mobile giants Candy Crush Saga and Clash of Clans.

What’s even more remarkable about the figures is that Fortnite is yet to arrive in the Google Play store. When it does, both player numbers and revenue are expected to increase again. Sensor Tower believes it could easily gross $3 billion per day once the number of Android users match their iOS counterparts.

The mobile version of Fortnite rival PUBG, meanwhile, is available on both Google’s and Apple’s platforms. It’s estimated to have even more iOS installs than Fortnite—14 million—though it’s yet to add in-app purchases.