Caffeine Half-Life Calculator
Find how much caffeine remains after a set time.
Average ~5 hours
Results update as you type.
This calculator provides estimates based on population research. Results are not medical advice. Consult a qualified healthcare provider before making health or nutrition decisions.
How it works
Caffeine lingers in your body longer than many people realize, and this calculator shows how much is left after a given time. You enter the caffeine dose, the hours elapsed, and the half-life, which defaults to about 5 hours, and it returns the caffeine remaining, the amount eliminated, and the percent remaining. Caffeine follows exponential decay: after each half-life, half of what was present is cleared, so the remaining amount equals the dose times one-half raised to the number of half-lives that have passed. With a typical half-life of 5 hours, a 200 mg dose, roughly two cups of coffee, leaves about 100 mg after 5 hours, 50 mg after 10, and 25 mg after 15. This is why an afternoon coffee can still affect sleep at bedtime. The half-life varies considerably between people: it can be shorter in smokers and longer, sometimes 8 hours or more, in pregnancy, with certain medications, or in people who metabolize caffeine slowly for genetic reasons. Adjust the half-life field if you know yours differs from average. Use the result to time your caffeine so it does not disrupt sleep, generally by avoiding it in the hours before bed. This is an educational estimate; individual responses to caffeine vary widely. Enter your dose, hours elapsed, and half-life to see how much caffeine remains.
Remaining = dose × 0.5^(hours / half-life)
Caffeine is cleared by first-order kinetics, so a fixed fraction leaves each half-life. The remaining amount is the dose multiplied by one-half raised to the number of half-lives elapsed, which is the hours divided by the half-life. After one half-life, half remains; after two, a quarter; and so on, decaying exponentially over time.
Source:First-order pharmacokinetics (caffeine metabolism)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter caffeine dose and hours elapsed.
- 2
Adjust the options
Adjust half-life if they apply — sensible defaults are already set.
- 3
Read your result
Caffeine remaining updates instantly as you type — no button to press.
Frequently asked questions
What is the half-life of caffeine?
The half-life of caffeine averages about 5 hours in healthy adults, meaning half of a dose is cleared in that time. It varies widely, though: it can be as short as 3 hours or as long as 8 or more depending on genetics, smoking, pregnancy, medications, and liver function. The calculator lets you adjust the half-life to match your situation.
How is caffeine remaining calculated?
Caffeine decays exponentially, so the remaining amount is the dose multiplied by one-half raised to the number of half-lives elapsed, which is the hours divided by the half-life. After one half-life half remains, after two a quarter, and so on. The calculator applies this formula to your dose, elapsed time, and half-life.
Why does afternoon coffee affect sleep?
Because caffeine's half-life is around 5 hours, a significant amount remains many hours after you drink it. A coffee at 3 p.m. can still leave a quarter of its caffeine in your system near midnight, enough to delay sleep onset and reduce sleep quality in sensitive people. Avoiding caffeine in the hours before bed helps protect sleep.
What makes caffeine half-life longer or shorter?
Half-life is shorter in smokers, who metabolize caffeine faster, and longer during pregnancy, with oral contraceptives and some other medications, with liver impairment, and in people whose genetics make them slow metabolizers. Newborns clear caffeine very slowly. If you know your metabolism differs from average, adjust the half-life field for a more personal estimate.
How much caffeine is safe per day?
For most healthy adults, up to about 400 milligrams a day, roughly four cups of coffee, is generally considered safe by health authorities. Pregnant people are usually advised to limit intake to around 200 milligrams. Sensitivity varies, so some people feel effects at lower doses. This calculator tracks how a dose clears, not a personalized safe limit.