Caffeine Half-Life Calculator

Caffeine follows first-order elimination kinetics: the amount remaining at time t equals C₀ × 0.5^(t / half-life). That's a straightforward exponential decay — every half-life period halves the remaining caffeine.

The default 5-hour half-life is the population average for healthy adults (Arnaud, 2011). But individual variation is enormous: the CYP1A2gene determines how quickly your liver clears caffeine. “Slow metabolisers” (about 50% of the population) have half-lives closer to 8 hours. Pregnancy roughly doubles half-life. Smoking shortens it. Hormonal contraceptives lengthen it.

We flag two thresholds: 100 mg (the level above which sleep architecture measurably degrades in lab studies — Drake et al, 2013) and 30 mg (roughly background level, usually negligible).

People consistently underestimate how long caffeine stays active. A 200mg dose at 2pm leaves roughly 50mg in your system at 10pm in an average metaboliser — enough to measurably reduce slow-wave sleep that night, even if you don't feel “wired” at bedtime.

If you suspect afternoon coffee is affecting your sleep, see caffeine and sleep and try the caffeine cut-off calculator.