Caffeine Half-Life Calculator
Caffeine is the world's most widely consumed psychoactive substance, found in coffee, tea, energy drinks, chocolate, and many medications. Once absorbed, caffeine follows predictable exponential decay: the body eliminates roughly half the amount every 5 to 6 hours in healthy adults. Knowing how much caffeine remains in your system at a given time helps you plan your last cup of the day to avoid disrupted sleep. Research shows caffeine within 6 hours of bedtime significantly reduces sleep quality. This calculator uses the standard pharmacokinetic half-life decay formula, letting you enter your dose in milligrams and the number of hours since consumption to see exactly how much caffeine is still active. It also shows a decay schedule across multiple intervals so you can plan accordingly.
Caffeine decay formula
Remaining (mg) = Dose x (0.5 ^ (t / half-life))
Where t is time elapsed in hours and half-life is the time for 50% elimination. For 200 mg at 3 hours with a 5.5-hour half-life: 200 x (0.5 ^ (3 / 5.5)) = 129.4 mg remaining.
Caffeine half-life: frequently asked questions
What is the half-life of caffeine?
The half-life of caffeine in healthy adults is approximately 5 to 6 hours. This means that 5-6 hours after consuming caffeine, roughly half the amount remains in your bloodstream. The FDA and NIH cite this range for healthy adults.
How does the calculator work?
The calculator uses the exponential decay formula: Remaining = Initial x (0.5 ^ (hours / half-life)). You input your caffeine dose and how many hours ago you consumed it, and the calculator shows how many milligrams remain active in your system.
What factors affect caffeine metabolism?
Liver enzymes (especially CYP1A2) metabolise caffeine. Genetics, smoking (speeds metabolism), pregnancy (slows it significantly - up to 15-hour half-life), oral contraceptives, certain medications, and liver disease all affect how quickly you process caffeine.
How much caffeine is in common drinks?
Approximate values: espresso (single shot) 63 mg, drip coffee (8 oz) 95 mg, black tea (8 oz) 47 mg, energy drink (8 oz) 80 mg, cola (12 oz) 35 mg. These vary by brand and preparation method.
What is a safe daily caffeine limit?
The FDA considers 400 mg per day safe for healthy adults. Pregnant individuals are advised to limit intake to 200 mg per day. Sensitivity varies considerably between people; some experience anxiety or disrupted sleep at much lower doses.
Official sources
- U.S. Food and Drug Administration: Spilling the Beans: How Much Caffeine Is Too Much?
- National Institutes of Health - MedlinePlus: Caffeine.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.