Caffeine calculator

How caffeine half-life works

Your body removes caffeine at a steady percentage rate, not at a steady number of milligrams. One half-life is the time your body needs to remove half of the caffeine. For most adults that is about five hours. Try one 95 mg coffee at 15:00 in the model above. It peaks near 74 mg at about 16:50. It is still around 53 mg at 20:00, 26 mg at 01:00, and 13 mg at 06:00. What is left at bedtime comes from that tail.

Absorption matters too. Caffeine does not enter your blood all at once. The level climbs for the first hour or two after you drink. The calculator models the climb and the decline with the same equation the app uses. A drink from 40 minutes ago counts for less than the same drink from two hours ago.

Caffeine clearance varies by person

Half-life is not the same for everybody. Clearance depends mostly on the liver enzyme CYP1A2. Measured half-lives in healthy adults span roughly two to ten hours. Smoking makes clearance faster. Oral contraceptives, pregnancy and some medicines make it slower.² Two people can drink the same flat white at the same hour and hold very different amounts at midnight. The 3-, 5- and 8-hour settings are scenarios to compare, not a test of your metabolism. Try more than one if you do not know your half-life.

Caffeine also matters later in the day than most people think. In a controlled trial, 400 mg of caffeine taken six hours before bed still cut total sleep time by a measurable amount.¹ Self-reports miss the loss: most participants in the study did not notice it. The disruption shows in the sleep recording first.

How the threshold and cutoff are calculated

WaterChamp uses 0.83 µg/mL as a model threshold, converted to milligrams with an assumed distribution volume of 0.7 litres per kilogram. That gives about 35 mg at 60 kg and 52 mg at 90 kg. The cited sleep study provides context for caffeine effects;³ it does not validate this setting as a universal safe-sleep cutoff. Being below the line does not guarantee unaffected sleep.

The next-drink cutoff searches from 04:00 until before bedtime, adding the caffeine amount you entered to your listed drinks. It finds a start time within five minutes of the latest modeled time below the threshold. The default is a 95 mg black coffee; the app reminder uses a 63 mg espresso, so their default cutoff times can differ. A result earlier than the current time is a planning result, not permission for another drink now.

The caffeine values in the drink list are the app’s defaults for one standard serving. A real cup varies with preparation; enter a label amount when you have one. The output is an estimate, not a measurement. Drinks still absorbing at bedtime can peak later, so a low bedtime value alone does not establish low exposure through the night.

Check the caffeine in your drink

Compare the serving examples for espresso, black coffee, cold brew, matcha latte and energy drinks. The drink guide lists the app's serving sizes and values together.

To work out your daily drinking target as well, use the water intake calculator.

In the app

The WaterChamp app runs the same curve on your phone from the drinks you log. It warns you before your last chance for coffee passes. It also tracks water, alcohol and calories from 102 pickable drinks. It writes your water intake to Apple Health and Health Connect.

Not medical advice. This is a general model, not a measurement of you. Half-life estimates vary widely between people and change with pregnancy, liver conditions, smoking and medicines. Talk to a doctor about caffeine if you are pregnant, taking medication, or having ongoing trouble sleeping.