Caffeine Half-Life Calculator
See how much caffeine is still in your system now, and how many hours until it drops below sleep-disrupting levels, from the dose, time and your half-life.
How it works
remaining = dose × 0.5^(hours ÷ half-life) · time to reach a threshold T = half-life × log₂(dose ÷ T)
Caffeine clears through first-order kinetics: every half-life, the amount in your blood halves. With an average half-life near 5 hours, 200 mg falls to 100 mg after 5 hours, 50 mg after 10, and so on. The half-life itself varies a lot — smoking roughly halves it, pregnancy and some medications can double it, and genetics shift it either way — which is why the same evening coffee wrecks one person’s sleep and not another’s. Rearranging the decay gives the time to drop below any level you care about, such as a sleep-safe threshold.
Worked example
A 200 mg dose 3 hours ago with a 5-hour half-life: 200 × 0.5^(3 ÷ 5) ≈ 132 mg still circulating, about 66% of the original. It passes below 100 mg at 5 hours after the coffee and below 50 mg at 10 hours.
Sources: Caffeine — clinical pharmacokinetics (StatPearls / NCBI)
Frequently asked questions
What half-life should I enter?
Around 5 hours is the healthy-adult average, but the real range is roughly 3–7 hours. Smokers clear caffeine faster (nearer 3 hours), while pregnancy, oral contraceptives and some medications can stretch it past 8–10 hours. If you are unusually sensitive to evening coffee, try a longer value.
How late is too late for caffeine before bed?
This is an estimate, not medical advice, but research links caffeine within about 6 hours of bedtime to worse sleep. Because a meaningful amount is still in your system for several half-lives, many people find an early-afternoon cutoff protects sleep. Sensitivity varies widely, so watch how your own sleep responds.
Does the amount ever reach exactly zero?
Not mathematically — exponential decay only approaches zero. In practice the last traces are negligible after about five half-lives (roughly 25 hours at a 5-hour half-life), which is why a strong afternoon coffee can still register the next morning.
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Educational estimate, not medical advice. Bodies differ — consult a qualified professional for health decisions.