Caffeine Half-Life Calculator

Written byOleh Soloninupdated 25 September 2026

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
From the dose to 50 mg9.6 h
From the dose to 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 calculated amount falls to the nominal marks of 50 and 25 mg — which do not guarantee it will not affect sleep.

Caffeine is cleared by first-order kinetics: every half-life leaves exactly half of what was there. The calculator defaults to five hours, which means a cup drunk at 3pm is still a little over a quarter present at midnight.

People differ widely. Smokers clear caffeine noticeably faster, women late in pregnancy several times more slowly, and oral contraceptives roughly double the half-life. That is why the half-life can be changed here.

How to use

Choose a drink and the number of servings — or enter the dose in milligrams if you know it.
Keep the 5-hour half-life or enter your own: shorter for smokers, longer in pregnancy or on oral contraceptives.
Enter how many hours have passed since the last cup.
The result gives how much caffeine is left and when the level drops below 50 and 25 mg.

Caffeine left hour by hour

Two americanos are 190 mg. After five hours half is left, after ten a quarter, after a day under 4 percent. That is why an afternoon coffee can keep you awake: a third of a 3pm cup is still there at 11pm.

A 190 mg dose, 5-hour half-life. Every row comes from the calculator on this page.
Hours passed	Left, mg	Share of dose
2	144	75.8%
4	109.1	57.4%
6	82.7	43.5%
8	62.7	33%
10	47.5	25%
12	36	18.9%
16	20.7	10.9%
24	6.8	3.6%

Caffeine in common drinks

One filter coffee is a quarter of the 400 mg daily guide. A cup of black tea or a can of cola starts below 50 mg, and an espresso drops under that level in less than two hours.

One serving, 5-hour half-life. Every row comes from the calculator on this page.
Drink	Caffeine, mg	Below 50 mg after	Share of 400 mg
Espresso, 30 ml	63	1.7 h	16%
Americano, 240 ml	95	4.6 h	24%
Filter coffee, 240 ml	95	4.6 h	24%
Instant coffee, 240 ml	62	1.6 h	16%
Energy drink, 250 ml	80	3.4 h	20%
Black tea, 240 ml	47	at once	12%
Cola, 330 ml	34	at once	9%
Green tea, 240 ml	28	at once	7%

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

The caffeine per drink is an editable example figure, not a measurement of your cup: it varies with the beans, the brewing method and the serving size. For an americano, count the espresso shots in it, or take the figure from the label.
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 body?

With a five-hour half-life, a quarter of the dose is left after ten hours and under 4 percent after a day. With a seven-hour half-life, 190 mg leaves 86 mg after eight hours instead of 62.7.

How many hours before bed should I stop drinking coffee?

Aim for six to eight. In Drake and colleagues' 2013 study, 400 mg of caffeine taken even six hours before bed cut sleep 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. A real cup can hold half or double that depending on the beans and the brew.

How much caffeine a day is safe?

EFSA and the FDA put it at up to 400 mg a day for healthy adults — about four cups of filter coffee. In pregnancy, no more than 200 mg.

Why does caffeine affect people so differently?

The half-life varies a lot: smoking shortens it, pregnancy and oral contraceptives lengthen it. With a three-hour half-life 190 mg drops below 50 mg in 5.8 hours; with nine hours it takes 17.3.

When should I have my last coffee if I sleep at 11pm?

Under the five-hour half-life model, 190 mg from two americanos falls to 50 mg after about 9 h 38 min, so by about 1:22pm; a single 95 mg cup takes about 4 h 38 min, so by about 6:22pm. That is the arithmetic, not a guarantee of undisturbed sleep.

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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