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Caffeine and water calculators
Four quick ways to see what your drinks do, worked out with the same model as the Caffeine vs Water app. No sign-up, nothing to install.
Calculator
Caffeine at bedtime
Add up to six drinks and see what's left at lights-out, hour by hour.
Calculator
When to stop drinking coffee
The latest time for your drink, for your bedtime and how caffeine hits you.
Calculator
How long caffeine stays in you
Hour by hour until it's gone, with the half-life that fits you.
Calculator
How much water you need
A daily goal from your weight, activity and climate. Coffee counts.
How they work
The caffeine tools use the standard one-compartment model from caffeine research: a drink is absorbed over about 45 minutes, then cleared at your half-life, 5.5 hours for a typical adult. The sleep line is 50 mg at bedtime, about half a cup of coffee. The water tool sets the same daily goal as the app: 33 ml per kg of body weight, plus more for activity and hot weather, plus a glass with each strong coffee.
They're estimates for healthy adults, not medical advice. The maths behind the calculator lists the studies.
Let the app keep count.
Caffeine vs Water does this all day: every drink added up, your usual coffee counted for you, and a heads-up before the one that would cost you sleep. For iPhone and Apple Watch, free in beta.
Needs an iPhone with iOS 26 or later. We pass your email to Apple to send the TestFlight invite. Privacy