RationAle gives you a rough guess at your blood alcohol content (BAC) based on the drinks you’ve had recently. Keep a list of your favourite drinks, or just enter the amount of standard drinks directly.

The app is especially handy when you've got an Apple Watch: your can add drinks from the watch, and the complication will show you your current estimated BAC. You can use ‘time travel’ to spin the crown and get a rough idea of how your BAC is going to change over time.

RationAle uses approximations based on the Widmark formula to make a guess at your blood alcohol content, based on your height, weight and sex—these guesses are intended to give you an idea of how your body might react to alcohol, and should not be used in place of an actual breathalyser, or to make serious decisions about when to drive.