Caffeine Half-Life Calculator

What "Half-Life" Actually Means Here

Caffeine leaves your body through first-order elimination kinetics — a fixed percentage clears every hour, not a fixed amount, so the remaining dose keeps halving on a regular schedule rather than dropping in a straight line. For the average healthy adult that schedule is around 5 hours: start with 200mg and 5 hours later roughly 100mg is still active, another 5 hours after that roughly 50mg remains, and so on. It never technically hits zero on this math, just gets small enough to be negligible after 4-5 half-lives.

How to Use This Calculator

Enter your caffeine dose in milligrams (a standard 8oz cup of coffee is roughly 95mg, an energy drink varies widely by brand, check the label) and how many hours have passed since you had it. The default 5-hour half-life is the commonly cited population average — adjust it if you have reason to think yours runs faster or slower.

How We Calculate It

Remaining caffeine = dose × 0.5^(hours elapsed ÷ half-life). For example: 200mg dose, 5 hours elapsed, 5-hour half-life → 200 × 0.5^(5÷5) = 200 × 0.5 = 100mg remaining.

A Few Notes

Individual half-life varies a lot — anywhere from about 1.5 to 9.5 hours depending on genetics (specifically CYP1A2 enzyme activity), liver function, smoking status, and other factors. Pregnancy can extend it dramatically, up to 15 hours in some cases, and hormonal contraceptives roughly double it. This is a population-average estimate for curiosity and rough planning (like figuring out if that afternoon coffee will affect tonight's sleep), not a precise measurement of your own metabolism, and it isn't medical guidance about safe caffeine intake.