Caffeine Half-Life Calculator
See how much caffeine is still in your system tonight, and when it falls below your sleep threshold.
- 2 cited sources
- Calculates in your browser
Your answer
Appears here as you enter your details. No demo figures — the number is yours, or it is nothing.
Baseline
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Get my Baseline →Methodology
Caffeine elimination follows first-order kinetics, a fixed proportion is cleared per unit time, so remaining caffeine is:
C(t) = dose × 0.5^(t ÷ t½)
The default half-life of 5 hours reflects the commonly cited average for healthy adults (Institute of Medicine review); the slider spans 1.5 to 9.5 hours because real half-lives vary that widely with genetics (notably CYP1A2 variants), smoking, pregnancy and medications such as some oral contraceptives. The chart plots the decay over 24 hours against your chosen sleep threshold, and the headline figure solves for when the curve crosses it.
Interpreting your result
The practical question is bedtime interference. A 200 mg mid-afternoon coffee still leaves ~50 mg circulating at a 5-hour half-life ten hours later, enough to measurably degrade sleep quality in sensitive people even when it doesn't stop you falling asleep. Work backwards: pick the threshold below which you sleep well (many people land somewhere near 50 mg), and the calculator tells you your effective caffeine curfew.
EFSA's safety assessment puts habitual intakes up to ~400 mg/day within safe limits for healthy adults. The issue this tool addresses is timing, not toxicity.
Limitations
- You rarely know your own half-life; if caffeine obviously disturbs your sleep, assume you sit at the slow end and set the slider high.
- Doses stack: the model shows a single dose, so a second coffee adds its own curve on top.
- Sensitivity to a given blood level also varies. The threshold is personal and found by experiment.
Sources
Pull the receipts, 2 cited sources
Frequently asked questions
- How long does caffeine stay in your system?
- The average half-life is about five hours, so a 200 mg coffee still leaves ~50 mg twelve to fifteen hours later. Genetics, pregnancy, smoking and some medications swing individual half-lives from under two to over nine hours.
- How much caffeine is in a coffee?
- Typically 80 to 120 mg per single espresso-based drink and 60 to 100 mg per instant cup, with large brewed coffees often exceeding 200 mg. Energy drinks usually carry 80 to 160 mg per can.
- When should I stop drinking caffeine before bed?
- With a 5-hour half-life, a 2 pm coffee still leaves a meaningful dose at 11 pm. Set your own threshold in the calculator; many people sleep noticeably better keeping evening levels below ~50 mg.
- Is caffeine bad for me?
- Moderate intake (up to ~400 mg/day for healthy adults, per EFSA's assessment) is considered safe for most people. Pregnancy and heart conditions change that calculus, so ask your doctor.
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