Caffeine Half-Life Calculator
Estimate how much caffeine remains in your body a given number of hours after a drink, using the average five-hour half-life, and when it falls below a level that could affect sleep.
How much of that coffee is still in you, hour by hour.
How the caffeine half-life calculator works
Caffeine is cleared exponentially: each half-life removes half of what remains. The average adult half-life is about five hours, so 200 mg at 2 pm is 100 mg at 7 pm, 50 mg at midnight and 25 mg at 5 am. The calculator applies remaining = dose × 0.5^(hours ÷ half-life) and also reports when the level drops under a threshold you choose.
Half-life varies a lot: pregnancy, the contraceptive pill and some medicines slow clearance to 8–10 hours or more, smoking speeds it up, and genetics matter. Detailed mode lets you set your own half-life.
Formula: remaining = dose × 0.5^(hours ÷ half-life)
Worked examples
| Inputs | Caffeine remaining | Note |
|---|---|---|
| 200 mg, 6 hours later | 87 mg | a little over one half-life |
| 200 mg, 10 hours later | 50 mg | two half-lives → a quarter left |
| 400 mg, 8 hours later | 132 mg | a big day of coffee |
| 95 mg, 3 hours, 8-hour half-life | 73 mg | slow metaboliser in Detailed mode |
FAQFrequently asked questions
How long does caffeine stay in your system?
With a five-hour half-life, about 6% of a dose is still present after 20 hours. Most people notice effects for 4–6 hours, but sleep studies find measurable disruption from caffeine taken six hours before bed.
How much caffeine is in a coffee?
Roughly 60–70 mg in an espresso shot, 90–120 mg in a 250 ml filter coffee, 30–50 mg in tea and about 80 mg in a small energy drink — but café servings vary widely.
Why is my half-life different?
Pregnancy and oral contraceptives roughly double it, some antibiotics and antidepressants slow it, smoking shortens it, and liver enzymes vary genetically. Set your own in Detailed mode.
Is there a safe daily limit?
Many health authorities cite about 400 mg a day for healthy adults and 200 mg in pregnancy. This calculator models clearance, not safety — check with a doctor if you have a heart condition or are pregnant.
Where these figures come from
- Mifflin MD et al., "A new predictive equation for resting energy expenditure in healthy individuals", Am J Clin Nutr 1990 — the BMR equation used here
- World Health Organization — Waist circumference and waist–hip ratio (2008 expert consultation) — waist-to-hip cut-offs
- Tanaka H, Monahan KD, Seals DR, "Age-predicted maximal heart rate revisited", J Am Coll Cardiol 2001 — 208 − 0.7 × age
- Nathan DM et al., "Translating the A1C assay into estimated average glucose values" (ADAG), Diabetes Care 2008 — eAG = 28.7 × A1c − 46.7
- Mosteller RD, "Simplified calculation of body-surface area", N Engl J Med 1987 — BSA = √(height × weight / 3600)
- Centers for Disease Control and Prevention — national health guidance
Last checked: September 2026. The equations are fixed by their published papers and do not change with tax years or regulations; the thresholds quoted in verdicts name the body that set them.