Caffeine Half-Life Calculator
Enter how much caffeine you had and how long ago to estimate how much is still in your system — handy for deciding whether that afternoon coffee will keep you up.
Caffeine leaves your body on a predictable curve. Its half-life — the time for half of it to be cleared — averages about 5 hours in healthy adults, so a dose keeps fading by half every 5 hours.
How the caffeine half-life calculation works
Every half-life, the amount left halves. To find what remains after any time, multiply the dose by one-half raised to the number of half-lives that have passed.
Remaining = Dose × 0.5 (hours ÷ half-life)
| Common source | Approx. caffeine |
|---|---|
| Brewed coffee (8 oz) | ~95 mg |
| Espresso (1 shot) | ~63 mg |
| Energy drink (8 oz) | ~80 mg |
| Black tea (8 oz) | ~47 mg |
| Cola (12 oz) | ~34 mg |
Worked example
200 mg of caffeine, 6 hours ago, at a 5-hour half-life:
| Half-lives passed = 6 ÷ 5 | 1.2 |
| Remaining = 200 × 0.5^1.2 | ≈ 87 mg |
Frequently asked questions
How long does caffeine stay in your system?
With a ~5-hour half-life, about a quarter of a dose remains after 10 hours and roughly an eighth after 15. It's largely cleared after about 5 half-lives (~24 hours), though sensitivity varies.
Why does the half-life vary?
Genetics, liver enzymes, pregnancy, some medications and smoking all change how fast you clear caffeine. Typical values range from about 3 to 7 hours.
How much caffeine is too much before bed?
Many people sleep better keeping caffeine under about 50 mg in the evening. Use the calculator to see when your dose drops below that.
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For general information and education only — not medical advice. Individual caffeine metabolism varies widely. Consult a healthcare professional with any health concerns.