Caffeine Half-Life
Calculator

Half-life is the time it takes your body to clear half of a substance. For caffeine it averages about 5 hours, but normal range is 3-7 hours depending on genetics, age, pregnancy, liver health, and some medications. After one half-life, 50% remains; after two, 25%; after five, about 3%. This calculator models exactly that exponential decay.

A typical 8 oz brewed coffee has about 95 mg, a single espresso shot 63 mg, black tea 47 mg, green tea 28 mg, cola 34 mg, and energy drinks 80-160 mg depending on the can. These are averages — real amounts vary with bean type, brew strength, and serving size. The calculator's drink library uses these standard values and lets you adjust any entry to match your actual drink.

A common guideline is to stop caffeine about 6-8 hours before bedtime, but the honest answer depends on your half-life and sensitivity. The calculator's "latest safe coffee" feature works backward from your target bedtime: it finds the latest time you could consume a drink while still dropping below your chosen threshold (50 mg by default) by the time you sleep. For a 5-hour half-life that usually lands 10-14 hours before bed for a full coffee.

Caffeine blocks adenosine receptors — the chemical that builds sleep pressure — and even moderate levels near bedtime delay sleep onset, reduce deep sleep, and shorten total sleep time. You may not feel "wired," but the sleep architecture is still affected. Research suggests caffeine consumed within 6 hours of bedtime can reduce total sleep by an hour or more, which is why timing matters as much as the amount.

Yes. The 5-hour figure is a population average. Slow metabolizers (CYP1A2 genetic variants) can run 6-8+ hours, while fast metabolizers clear caffeine in 3-4 hours. Pregnancy roughly doubles half-life, oral contraceptives slow it, and smoking speeds it up. If you notice caffeine affecting you unusually long, try the 7-hour setting; if it barely affects you, try 3 hours.

No. Everything is computed locally in your browser with plain JavaScript. Your intake log is only saved to your own browser's local storage (so it survives reloads) and is never uploaded, transmitted, or shared anywhere. There is no account, no tracking, and no server involved.