Caffeine Half-Life Calculator

Drink

Servings

Your own dose, mg

Leave at 0 to use the drink.

Half-life, h

Around 3 h in smokers, up to 15 in pregnancy.

Hours elapsed

Result

Left, mg62.7
Left, %33%
Taken, mg190
Time to reach 50 mg9.6 h
Time to reach 25 mg14.6 h
Share of the 400 mg daily reference, %48%

This result is informational only and is not professional advice.

Half of it is gone every five hours

Works out how much caffeine is left after a given number of hours and when the level falls below the point that interferes with sleep.

Caffeine clears by first-order kinetics: every half-life leaves exactly half of what was there. For most adults that half-life is about five hours, so a cup at three in the afternoon still has roughly a quarter of its caffeine in circulation at midnight.

Individual variation is wide. Smokers clear caffeine about twice as fast, pregnancy slows it several-fold, and oral contraceptives roughly double the half-life. That is why the half-life here is an input rather than a constant.

What this calculator shows

The milligrams of caffeine left after the hours you enter
How long until the level drops below 50 mg and below 25 mg
What share of the 400 mg daily reference your dose represents

What to keep in mind

Caffeine content per drink is an average: a real cup varies with bean, grind and brewing method, sometimes by a factor of two.
The 400 mg daily figure is a reference for healthy adults. Pregnancy guidance is 200 mg, and children and adolescents considerably less.

FAQs

How long does caffeine stay in your system?

The half-life is around five hours and full clearance takes roughly a day. After ten hours about a quarter of the dose is still there.

How long before bed should I stop drinking coffee?

Six to eight hours is the usual advice. In one study, 400 mg taken six hours before bed cut sleep duration by more than an hour.

How much caffeine is in a cup of coffee?

About 63 mg in an espresso, roughly 95 in a 240 ml americano or filter coffee, 47 in black tea, and 80 in a 250 ml energy drink.

How much caffeine a day is safe?

For healthy adults the reference is up to 400 mg, about four cups of coffee. In pregnancy the recommendation is no more than 200 mg a day.

Worked example

Two americanos in the afternoon

Input: 2 americanos (190 mg), 5-hour half-life, 8 hours elapsed

Output: 62.7 mg left — a third of the dose

Note: It takes 9.6 hours to reach 50 mg and 14.6 to reach 25. If you go to bed at eleven and are caffeine-sensitive, the last cup belongs before midday.

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Caffeine Half-Life Calculator

Result

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