Caffeine Half Life Calculator

Caffeine half life tells you how long a dose stays active. The amount halves every five hours or so, which is why an afternoon coffee can still be in your system at bedtime.

DoseThe amount of caffeine consumed, in milligrams.Half lifeHours to remove half. About 5 for a healthy adult, longer in pregnancy.Hours since the doseHow long ago the caffeine was consumed, in hours.Target levelThe level you want to fall below, in milligrams. 50 is about half a cup.

Caffeine remaining

65.9754

Percentage remaining: 32.99%
Hours until the target: 10
Hours to clear 90 percent: 16.61
Half lives elapsed: 1.6

Values update as you type. This calculator covers the single scenario its formula assumes — see Common Mistakes for what it leaves out.

The formula this calculator uses

remaining = dose x 0.5 ^ (hours / half life)

remaining: The caffeine still in your body, in milligrams
dose: The amount consumed, in milligrams
half life: About 5 hours in healthy adults
hours: Time since the dose was taken

How to check the result by hand

1
Write down the dose and the half life
A 200 milligram coffee with a 5 hour half life. Healthy adults sit near 5 hours, but pregnancy and some medicines extend it.
2
Find the number of half lives
8 hours divided by 5 is 1.6 half lives. This ratio, not the raw hours, is what drives the decay.
3
Raise one half to that power
0.5 to the power 1.6 is 0.329877, so about a third of the dose is still active after 8 hours.
4
Multiply by the dose
200 times 0.329877 is 65.9754 milligrams. Reaching 50 milligrams takes 10 hours, and clearing 90 percent takes 16.61 hours.

For worked examples, common mistakes and the limits of this formula, read the full How To Calculate Caffeine Half Life page.