A drink-tracking assistant for iPhone and Apple Watch.

RationAle will give 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.

You can add drinks from the Apple Watch app as well, and the complication will show you your current estimated BAC. You can use ‘time travel’ to 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 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.

What's a Widmark?

The general concept of the Widmark formula is outlined on the Blood alcohol content wikipedia page. I've used this approach with some updated Widmark constants as suggested by Barbour and Forrest, as cited by John Searle:

$$w_{women} = 0.9367 - 0.01240 × BMI$$ $$w_{men} = 1.0181 - 0.01213 × BMI$$

...where your BMI is \(\frac{weight\ in\ kg}{height\ in\ metres^2}\). This can then be plugged into the Widmark formula:

$$ massOfAlcohol = volume × percentOfAlcohol × 0.789 g/ml $$ $$ alcoholEliminationRate = \frac{7.5 g}{60 m} $$ $$ BAC = \frac{100 × (massOfAlcohol - alcoholEliminationRate × timePassed)}{w × mass} $$

Tom Charman