This caffeine calculator tracks your dose through time: enter how much caffeine you had and when, and see what remains in your system hour by hour — including the number that decides your night, how much is still aboard at bedtime.
| Hours after coffee | Caffeine remaining |
|---|
Half-Life: The Only Caffeine Math There Is
Your liver clears caffeine exponentially: remaining = dose × ½(hours ÷ half-life). Half-life averages ~5 hours in adults but honestly ranges 3–10: smoking speeds it, pregnancy and oral contraceptives slow it dramatically, and CYP1A2 genetics deal everyone a different hand — hence the selector. The sleep connection is the point of the exercise; pair this with our Sleep Calculator to schedule the other end of the night. Part of our Health Calculators collection.
Worked Example (Verified)
200 mg (a strong 16 oz coffee) at a 5-hour half-life:
- After 5 h: 100 mg · after 6 h: 87 mg · after 10 h: 50 mg — a full espresso’s worth, still working at hour ten
- Bedtime audit: coffee at 8 AM, bed at 10 PM (14 h) → 200 × ½^2.8 ≈ 29 mg — mild. The same coffee at 2 PM → 8 hours → 66 mg — the difference between the two habits is one full sleep-quality grade
- Research anchor: a well-known 2013 study (Drake et al.) found 400 mg taken even 6 hours before bed cut measured sleep by ~1 hour — subjects often didn’t perceive the loss.
Dose Reference
| Drink | Caffeine ≈ | Drink | Caffeine ≈ |
|---|---|---|---|
| Espresso shot | 63 mg | Black tea (8 oz) | 47 mg |
| Drip coffee (8 oz) | 95 mg | Green tea (8 oz) | 28 mg |
| Cold brew (16 oz) | 200 mg | Cola (12 oz) | 34 mg |
| Energy drink (16 oz) | 160 mg | Dark chocolate (1 oz) | 24 mg |
Related Tools
- Sleep Calculator — plan the night itself
- Health Calculators — the full collection
Frequently Asked Questions
How long does caffeine stay in your system?
With a ~5-hour half-life: half remains at 5 hours, a quarter at 10, and an eighth (~12%) at 15 hours. “Gone” for practical purposes takes 5 half-lives — about a full day for slow metabolizers after a big dose.
How much caffeine is in coffee?
Roughly 95 mg per 8 oz of drip coffee, 63 mg per espresso shot, and up to 200+ mg in a large cold brew. Café servings and bean varieties swing these by ±40%.
Will an afternoon coffee ruin my sleep?
Depends on dose and your half-life: 200 mg at 2 PM leaves ~66 mg at a 10 PM bedtime (5 h half-life) — enough to measurably lighten sleep in studies, especially for slow metabolizers. The calculator lets you test your own pattern.
What is a safe daily caffeine limit?
FDA guidance cites 400 mg/day as an amount not generally associated with negative effects in healthy adults; pregnancy guidance typically halves that to ~200 mg. Personal sensitivity rules below any ceiling.
Why does caffeine affect people so differently?
Half-life varies 3–10+ hours via genetics (CYP1A2), hormones, pregnancy, medications and smoking — plus adenosine-receptor sensitivity differs. Two people, same coffee, genuinely different drugs by midnight.
Is this caffeine calculator free?
Yes — free, instant, no sign-up.