Caffeine Clearance Time Calculator
See how long caffeine lingers in your body: when your dose falls below a sleep-safe level and when it is essentially gone.
How it works
Remaining = dose × 0.5^(t ÷ half-life) · Time to level = half-life × log2(dose ÷ level)
Caffeine clears through first-order (exponential) decay: a fixed fraction leaves each hour, so the amount halves once per half-life. To find when your dose drops to a chosen level you invert that: multiply the half-life by the base-2 logarithm of the ratio of dose to level. The average half-life is around five hours but varies a lot — it can double in pregnancy or with certain medications and shorten in regular smokers.
Worked example
With a 200 mg dose and a 5-hour half-life, reaching a 50 mg sleep-safe level takes 5 × log2(200 ÷ 50) = 5 × 2 = 10 hours. After 6 hours you still have 200 × 0.5^(6 ÷ 5) ≈ 87 mg on board.
Frequently asked questions
Why does caffeine keep me awake so long?
With a ~5-hour half-life, an afternoon coffee can still leave a meaningful dose in your bloodstream at bedtime. Even sub-threshold caffeine can lighten sleep, which is why cutting off caffeine 8–10 hours before bed helps many people.
What changes my caffeine half-life?
Genetics, liver enzyme activity, pregnancy, oral contraceptives and some medications can lengthen it, while regular smoking shortens it. Set the half-life field to reflect your situation if you know it differs.
Is caffeine ever fully at zero?
Exponential decay approaches zero without ever mathematically reaching it, so this tool reports the practical point where under ~10 mg remains — an amount most people would not notice.
Related calculators
Educational estimate only — not medical advice, diagnosis, or treatment. Individual needs vary; consult a qualified healthcare professional for decisions about hydration, nutrition, medication, or substance use.