The iOS App Store’s sales charts had already suggested that Fallout Shelter was a success, but now we have a more specific idea of how well it’s done.

According to research firm SuperData, Shelter earned $5.1 million in its first two weeks of availability alone. While Apple does get a 30 percent cut, that’s still a very impressive figure, particularly considering Bethesda has said Shelter wasn’t designed in order to make money.

And, indeed, as anyone who’s played the game can tell you, the free-to-play game’s microtransactions are limited to one specific aspect of the game. You’re not restricted from playing as much as you want, and the only things to spend money on are optional lunchboxes, which reward you with in-game items and characters. You even earn these occasionally by simply playing, and it’s no doubt thanks in part to this player-friendly approach that the game has been so successful.

Announced and then released at the start of E3 week, Fallout Shelter quickly rose to the number one spot on both the Top Free and Top Grossing charts of the iOS App Store. Currently, the game is available only on iOS, though an Android version is on the way.

Last month, Shelter added its first Fallout 4 character. Since then, it’s also received its first feature update, which introduced a photo mode that lets you easily capture and share images of the Vault you’ve designed.

Fallout 4, the next proper entry in the Fallout series, is due out in November for Xbox One, PS4, and PC.