Caffeine Half-Life Calculator
Caffeine clears via first-order pharmacokinetics — every ~5 hours (the average adult plasma half-life) the amount in your bloodstream halves. Enter the dose in mg, the time you drank it and your planned bedtime to see how much is still on board at sleep time, when it crosses the 100 / 50 / 10 mg thresholds, and an hour-by-hour breakdown for the next 24 hours. Useful for figuring out your personal "no coffee after ___" rule.
Enter positive numbers for dose and half-life.
Remaining at bedtime
— mg
—
Time to drop below…
- below 100 mg
- —
- below 50 mg
- —
- below 10 mg (trace)
- —
Hour-by-hour over the next 24 h
| Time | Remaining (mg) | % of dose |
|---|
Formula
C(t) = D · (1/2)^(t / t½) | t = t½ · log₂(D / target) | default t½ = 5 hours
- · The 5-hour default is the population median for healthy adults (FDA Caffeine pharmacology monograph; Institute of Medicine, 2001).
- · Half-life varies hugely: CYP1A2 "fast metabolisers" can clear in ~3 h; women on oral contraceptives extend to ~9 h; late pregnancy 10–15 h; severe liver disease up to 96 h.
- · Typical doses: espresso shot ≈ 80 mg, drip coffee ≈ 95–165 mg/cup, Starbucks Grande latte ≈ 150 mg, 250 mL Red Bull ≈ 80 mg, 473 mL Monster ≈ 160 mg, can of Coca-Cola (330 mL) ≈ 32 mg, pre-workout scoop ≈ 150–300 mg.
- · Sleep research (Drake et al., J Clin Sleep Med 2013) shows ≥ 100 mg consumed within 6 hours of bedtime measurably reduces total sleep time and subjective sleep quality.
- · This tool uses a single-compartment first-order elimination model and ignores the ~30–45 min oral absorption phase, so values in the first hour after intake slightly under-estimate plasma concentration.
- · FDA guidance for healthy adults is ≤ 400 mg/day total caffeine; ≤ 200 mg/day during pregnancy. Multiple doses are not summed here — add them yourself if you drink more than one.
Frequently asked
Why does coffee not bother my friend but keeps me awake all night?
Mostly genetics. ~95 % of caffeine is metabolised in the liver by the CYP1A2 enzyme, and the CYP1A2 gene has two common variants: AA (fast metaboliser) gives a half-life as short as 3 h, while AC/CC (slow metaboliser) extends it to 7–9 h. The ADORA2A adenosine-receptor gene further controls how sensitive you feel to a given level. Try setting the half-life to 7–8 h in the tool and watch the bedtime number jump — that is why an afternoon coffee can wreck an 11 pm bedtime for one person but not another.
If I want under 50 mg by bedtime, when is the latest I can drink a 200 mg coffee?
Use t = t½ · log₂(D / target). For 200 mg with a 5-hour half-life, dropping to 50 mg takes 5 × log₂(200/50) = 5 × 2 = 10 hours. If you go to bed at 11 pm you must finish that cup by 1 pm. A slow metaboliser (7 h half-life) needs to stop by 11 am; a fast metaboliser (3 h) can push it to ~5 pm. Adjust the dose and half-life in the tool and the "below 50 mg" row updates instantly.
How long until caffeine is completely cleared?
Pharmacologically, "complete clearance" is usually 5 half-lives, at which ~3 % of the dose remains. For an average 5-hour half-life that is 25 hours — meaning your noon coffee is not fully gone until noon tomorrow. This is why daily caffeine accumulates: yesterday's tail is still around when you start today. With 200 mg at 8 am every day and a 6-hour half-life, by day 5 you wake up with ~6 mg already on board before the first sip.
Do exercise or alcohol change caffeine half-life?
Exercise alone has little effect — only ~5 % of caffeine leaves via urine; the rest is metabolised in the liver. Two interactions matter, though: (1) grapefruit juice inhibits CYP1A2 and can extend the half-life by ~30 %; (2) smoking induces CYP1A2 and shortens half-life to ~3.5 h, which is why coffee feels stronger after quitting smoking. Alcohol shares the liver pathway and heavy drinking slows caffeine clearance, but one or two drinks have a small effect.
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