Caffeine half-life calculator
Caffeine has a half-life of about five hours in a typical adult, so an afternoon coffee is still a quarter present at midnight.
A drip coffee is around 95 mg, an espresso 63 mg, a can of cola 34 mg.
Typical adults 4 to 6 hours; smokers nearer 3; pregnancy can double it.
- Fraction of the dose
- 43.5 %
- Hours until under 25 mg
- 9.6
Show the working
- Caffeine taken = 95
- Hours since = 6
- Your half-life = 5
- Caffeine still in you = 95 * 0.5^(6 / 5) = 41.3512
- Fraction of the dose = 41.3512 / 95 * 100 = 43.5275
- Hours until under 25 mg = 95 > 25 ? 5 * log(95/25, 2) : 0 = 9.63
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- remaining = dose x (1/2)^(hours / half-life)
Half-life varies enormously between people: oral contraceptives roughly double it, smoking roughly halves it.
This is a population formula, not a diagnosis. Individual metabolism, body composition and medical history all move the real answer, so treat the number as a starting point and talk to a clinician about anything that matters.
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