Caffeine half-life calculator

With the average 5-hour half-life, half of a dose is still in your body after 5 hours, a quarter after 10 and about 3% after 25, so 200 mg of coffee at 08:00 leaves 50 mg at 18:00. Half-lives in healthy adults range from about 1.5 to 9.5 hours (Antonio et al., Journal of the International Society of Sports Nutrition, 2024), so the same cup can last several times longer in one person than in another.

For most healthy adults the FDA cites 400 mg a day, about two to three 12 fl oz cups of brewed coffee, as an amount not generally associated with negative effects. The FDA estimates that toxic effects such as seizures can appear after rapidly consuming about 1,200 mg. The calculator warns when the doses you enter add up to more than 400 mg.

At least 8.8 hours for a 107 mg cup of coffee and 13.2 hours for a 217.5 mg pre-workout drink, according to a 2023 meta-analysis of 24 studies in Sleep Medicine Reviews (Gardiner et al.). Across those studies caffeine cut total sleep by 45 minutes and added 9 minutes to the time taken to fall asleep. In an earlier trial, 400 mg taken 6 hours before bed still disrupted sleep (Drake et al., 2013).

The FDA gives 113 to 247 mg for 12 fl oz of brewed coffee, 71 mg for black tea, 37 mg for green tea, 23 to 83 mg for soft drinks and 41 to 246 mg for energy drinks of the same size; 8 fl oz of decaf has 2 to 15 mg. Brewing method and cup size change the dose, so use the label figure where there is one.

Less than 200 mg a day. The American College of Obstetricians and Gynecologists' Committee Opinion 462 states that intake below 200 mg a day does not appear to be a major contributing factor in miscarriage or preterm birth. Caffeine is also cleared more slowly: one study measured a mean half-life of 8.3 hours in pregnant women against 3.4 hours in other adults (Knutti et al., 1981), so a longer half-life gives a more realistic curve.

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 5 worked examples whose answers come from independent sources; for example, “200 mg at 08:00, checked at 18:00 (two half-lives)” is checked against Python 3.8 decimal: 200 × 0.5^(10/5) = 50; threshold reached at 5 × log2(200/50) = 10 h.

Institute of Medicine (US) Committee on Military Nutrition Research. Caffeine for the Sustainment of Mental Task Performance (2001) — pharmacology of caffeine; FDA — Spilling the beans: how much caffeine is too much?; Antonio J et al. Common questions and misconceptions about caffeine supplementation. J Int Soc Sports Nutr 2024;21:2323919 (mean half-life about 5 h, range 1.5–9.5 h); Gardiner C et al. The effect of caffeine on subsequent sleep: a systematic review and meta-analysis. Sleep Med Rev 2023;69:101764.