Caffeine Half-Life Calculator
Find out how much caffeine is still in your body after a few hours, using the roughly 5-hour half-life. Enter the dose and the time elapsed.
Caffeine (mg)
—
Caffeine remaining after n hours
Your body clears caffeine through first-order elimination, which the formula C(t) = C₀ · (1/2)^(t / t½) captures, where C₀ is the dose, t is the hours that have passed, and t½ is the half-life. Most healthy adults sit around a 5 hour half-life, though it ranges from 3 to 7 h. Take 200 mg at 18:00 with a 5 h half-life and you still have roughly 100 mg at 23:00, about 50 mg at 04:00, and ~25 mg by 09:00. Biology moves that half-life around a lot. Pregnancy stretches it to 9–13 h, clearing caffeine three times slower; oral contraceptives roughly double it; smoking speeds it up to 3–4 h through CYP1A2 induction; and a CYP1A2 polymorphism (AA vs AC/CC) separates adults into fast and slow metabolizers. Newborns need ~80 hours to clear a dose because their liver enzymes haven't matured yet, and kids catch up to adult rates around puberty.
Practical uses
Time a pre-workout dose properly (plasma peaks around 45–60 min, with the ergogenic window running 1–3 h), check whether that afternoon coffee will still be hanging around at bedtime (aim for under 50 mg when you fall asleep), figure out what's feeding your insomnia, or pace anti-fatigue doses across a long drive so you never spike too high. The sneaky problem is stacking: drink more before the last dose has cleared and your blood level climbs into jittery, heart-racing territory before you realize what happened.
FAQ
How do I know my half-life? Short of a genetic test, go with 5 h. Use 4 h if you smoke, or 9–13 h if you're pregnant.
When is caffeine "fully gone"? After roughly 5 half-lives, which is about 25 h for an average adult, only ~3% is left, so call it zero.
Does tolerance change the math? No. Tolerance affects how you feel, not how quickly your liver works. Someone who drinks coffee every day still has 100 mg in their blood at hour 5.
Why does coffee hit harder on an empty stomach? It absorbs faster, peaking in around 30 min instead of the 60–90 min you'd see with food. The total caffeine doesn't change.
Related Tools
Caffeine Calculator
Calculate how long caffeine stays in your body. See the remaining amount after each half-life and when it is safe to sleep. Includes a sources table (coffee, tea, energy drinks, chocolate). Everything in your browser.
Number of Half-Lives
Compute how many half-lives have elapsed, n = t/T½, and the fraction of radioactive material remaining, (1/2)ⁿ, from the elapsed time and the half-life. With each half-life the amount halves: after 1 half-life 50% remains, after 2 it is 25%, after 10 less than 0.1%. It is the intuitive way to assess how much of a source (or contamination) is left. Enter the elapsed time and the half-life.
Caffeine Decay
Estimate remaining caffeine after t hours assuming 5h half-life.
Radioactive Half-life
Compute remaining amount A = A₀·(1/2)^(t/T) for radioactive decay.
Effective Half-Life
Compute the effective half-life of a radionuclide in the body, 1/Teff = 1/Tbio + 1/Tphys, combining the physical half-life (radioactive decay) with the biological one (elimination by the organism). Since both processes reduce the amount simultaneously, the effective half-life is always shorter than either. It is essential in internal dosimetry in nuclear medicine and radiation protection. Enter the biological and physical half-lives.
Isotope Half Life Time
Computes remaining mass of a radioactive isotope after N half-lives using decay law.
The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.