Caffeine Half-Life Calculator
How it works
Caffeine clears the body by exponential decay: remaining = dose × 0.5 ^ (hours elapsed ÷ half-life). Every half-life that passes halves whatever amount is currently left, rather than removing a fixed amount per hour — which is why the remaining amount keeps dropping but never quite reaches zero.
The default half-life, 5.5 hours, is a commonly cited average for a healthy adult. It varies significantly by person: pregnancy or certain medications can roughly double it, while other factors can shorten it. Enter your own figure if you have a reason to think yours differs.
This is an informational estimate, not medical advice — it doesn't account for your individual metabolism, and shouldn't be used to judge how caffeine will affect you specifically.
Frequently asked questions
Why does half-life vary so much between people?
Genetics, liver function, pregnancy, smoking status, and certain medications all measurably affect how fast the body clears caffeine — some of these can roughly double or halve it. The 5.5-hour default here is a population average, not a personal number, so treat it as a starting point rather than your own rate.
What does "half-life" actually mean?
It’s the time it takes for HALF of whatever caffeine is currently in your system to clear — not a fixed amount per hour. Because each half-life only removes half of what’s left, a small amount is technically always still present; it just becomes negligible after a few half-lives.
Is 25 mg a guaranteed safe amount for sleep?
No — it’s a commonly cited rough guideline for an amount unlikely to disrupt sleep for most people, not a guarantee for any specific person. Sensitivity to caffeine varies, and factors like your own metabolism, sleep pressure, and how much caffeine you’re used to all matter too.