Health Calculators
Caffeine half-life.
See how much caffeine is still in your body hours after a drink, and how long until it falls to a level you choose. Exponential decay, with a half-life you set. Runs in your browser; nothing is sent anywhere.
Your drink
Caffeine remaining
Fill the form and press Calculate.
Decay over the day
Worked example
You drink one 8 oz brewed coffee (135 mg, Health Canada) and use a 5 hour half-life. After 6 hours: 135 × 0.5(6/5) = 135 × 0.435 = 58.8 mg, or 43.5% of the original. To find when it drops to 25 mg: 5 × log2(135/25) = 12.2 hours.
How it works
remaining = dose × 0.5(time / half-life). After one half-life half is left, after two a quarter, and so on. The time to reach a target amount is half-life × log2(dose / target). This assumes simple first-order elimination from a single dose, taken all at once and fully absorbed. It ignores absorption time, food, and caffeine already in your body from earlier drinks (treat them as a separate calculation).
Half-life is personal. The default of 5 hours is not a rule. A clinical study reported a mean caffeine elimination half-life of 5.0 hours in men and 9.7 hours in women taking oral contraceptives (PubMed 15060505, Clin Pharmacol Ther 2004), and another reported 5.8 hours in 8 healthy male volunteers (PubMed 2612543, Eur J Clin Pharmacol 1989). Some medicines slow clearance; in that second study, terbinafine raised it to 7.6 hours. Change the half-life to suit.
Drink amounts are published averages (as of 2026-10-01). Real drinks vary widely, so check the label. 12 fl oz figures are from the FDA (updated 2024-08-28): brewed coffee 113 to 247 mg, black tea 71 mg, green tea 37 mg, caffeinated soft drinks 23 to 83 mg, energy drinks 41 to 246 mg. Health Canada (page modified 2025-04-02) lists 135 mg for 8 oz of brewed coffee and 8 mg for 8 oz of chocolate milk.
Daily limits. Health Canada: up to 400 mg a day for adults, 300 mg a day if pregnant, planning pregnancy or breastfeeding, and 2.5 mg per kg of body weight for children and adolescents. The FDA cites 400 mg a day for most healthy adults.
Sources (as of 2026-10-01): Health Canada, Caffeine in foods; FDA, Spilling the Beans: How Much Caffeine Is Too Much?; PubMed 15060505; PubMed 2612543.
FAQ
What is caffeine's half-life?
The time it takes the body to clear half of the caffeine in it. Studies of healthy adults report a mean of about 5 hours (5.0 h in men in one study, 5.8 h in healthy controls in another), but it varies widely between people.
Why can I change the half-life?
Published values are averages from small studies, and individuals differ. In one study the mean was 9.7 hours in women taking oral contraceptives against 5.0 hours in men. Set the half-life that fits what you know about yourself.
How much caffeine is too much?
Health Canada gives a maximum of 400 mg a day for adults, 300 mg a day if pregnant, planning pregnancy or breastfeeding, and 2.5 mg per kg of body weight for children and adolescents. The FDA also cites 400 mg a day for most healthy adults.
Does this tell me when it is safe to sleep or drive?
No. It is a simple decay estimate, not a measurement of the caffeine in your blood or of how it affects you. Sensitivity differs, so treat the number as a rough guide only.
Track your fluids with the water intake calculator, and see the jet lag calculator if you are crossing time zones.
Disclaimer
Not medical advice and not a dosing tool. This is a simple estimate; it does not measure caffeine in your body and cannot say when it is safe to sleep, drive or take any medication. Talk to a healthcare professional about caffeine if you are pregnant, have a heart condition, or take medicines.