HealthScorer

Caffeine half-life calculator

Calculate when caffeine clears your system based on half-life, intake time, smoking, contraceptives and pregnancy. Sleep-friendly cut-off and.

Last updated: Sources verified:

What the calculator estimates

You input how much caffeine you took, when you took it, and a few personal factors. The model runs first-order pharmacokinetics:

C(t) = dose × 0.5^(t / half-life)

Default half-life is 5 hours — the population average for healthy adults (Arnaud 1987). Modifiers shift the half-life:

FactorEffect on half-life
Smoker× 0.5 (≈ 2.5 h)
Combined oral contraceptive× 1.5 (≈ 7.5 h)
Pregnancy (3rd trimester)× 2.5 (≈ 12.5 h)
Severe liver impairment× 2.0 (≈ 10 h)

The calculator returns:

  • Your current caffeine level at “now”
  • The time it drops below 100 mg — a practical sleep threshold
  • The time it drops below 10 mg — essentially cleared
  • A simple decay chart so you can see the curve

Why “5 half-lives = cleared” matters

After 5 half-lives, only ~3% of the original dose remains — small enough that pharmacological effects are negligible for most people. For a typical adult that is ~25 hours from the last cup. For a slow metabolizer (genetic CYP1A2 variant or pregnancy), 5 half-lives can stretch to 50+ hours.

This is why “I had one coffee yesterday and slept badly tonight” can be physiologically real, not just imagined.

Caffeine in common sources

Drink / itemTypical dose
Espresso shot (30 mL)60–80 mg
Filter coffee (240 mL)95–165 mg
Instant coffee (240 mL)60–100 mg
Black tea (240 mL)40–70 mg
Green tea (240 mL)25–45 mg
Cola (330 mL)30–40 mg
Energy drink (250 mL)80–110 mg
Pre-workout (1 scoop)150–300 mg
Dark chocolate (50 g)25–40 mg

These vary by brand, brew time, and bean. When in doubt, check the label or the SCA reference.

Sleep-friendly rule of thumb

Drake 2013 showed measurable sleep disruption from caffeine consumed up to 6 hours before bed. A practical translation:

  • Bedtime 23:00 → last dose by 14:00–15:00 for normal metabolizers
  • Bedtime 22:00 → last dose by 13:00–14:00
  • Slow metabolizer or pregnancy → consider stopping by noon or switching to decaf

You can also test your own response: cut caffeine after 14:00 for two weeks and track sleep quality with our Sleep cycle calculator and the Insomnia Severity Index.

Limitations

  • Genetic variation in CYP1A2 can shift half-life by 2–3× independent of the modifiers above. If you metabolize very fast (no jitters from late espresso) or very slowly (one morning coffee = bad sleep), the model is a starting point — your own response is the data that matters.
  • Drug interactions (fluvoxamine, ciprofloxacin, mexiletine, some antidepressants) sharply inhibit CYP1A2 and extend caffeine half-life. The calculator does not model these — flag them with your prescriber.
  • Children, severe cardiovascular disease, anxiety disorders — caffeine recommendations differ. Use clinical guidance.

Privacy

All calculations run in your browser. We never see, log, or store the values you enter. Only an anonymous event is sent to a privacy-respecting analytics service.

Frequently asked questions

How long does caffeine actually stay in your system?
Half-life — the time for blood concentration to drop by half — averages 5 hours in healthy adults but ranges 1.5–9.5 hours individually (Arnaud 1987). After roughly 5 half-lives, ~97% has cleared. So a 200 mg coffee at noon means ~6 mg at midnight in an average metabolizer, ~25 mg in a slow one, and ~1 mg in a fast one. The calculator estimates *your* curve from the modifiers you select.
Why does smoking shorten caffeine half-life?
Cigarette smoke induces the liver enzyme CYP1A2, which metabolizes caffeine. Smokers clear caffeine roughly twice as fast (half-life ~2.5 hours). This is why people often notice they need more coffee to feel the same effect when smoking, and may experience caffeine withdrawal headaches in the first days after quitting — the same dose now lasts twice as long.
Why does pregnancy or oral contraceptives slow it down?
Estrogens inhibit CYP1A2. Combined oral contraceptives roughly extend caffeine half-life by ~1.5×. Pregnancy is more dramatic: by the third trimester, half-life can reach 15 hours — about 3× baseline. EFSA 2015 recommends pregnant women keep total caffeine below 200 mg/day; the calculator uses a 2.5× modifier as a population midpoint.
What time should I stop drinking coffee for sleep?
Drake et al. 2013 found that caffeine consumed even 6 hours before bed measurably disrupted sleep quality. The calculator returns the time your remaining caffeine drops below 100 mg — a practical threshold below which sleep impact is small for most adults. As a default heuristic: no caffeine after 14:00 if you go to bed at 23:00.
Does the calculator account for tolerance?
Tolerance affects subjective alertness, not pharmacokinetics. Daily users feel less buzz from the same dose, but the half-life and clearance are essentially unchanged. So sleep disruption from a late-afternoon coffee can still happen even if you 'feel fine' — your brain may have adapted, but the molecule is still in your system.
Does my data leave the device?
No. All calculations run entirely in your browser. Only an anonymous event (your locale and modifiers) is sent to a privacy-respecting analytics service.

Sources

  1. Caffeine pharmacokinetics — clinical pharmacology review — Pharmacology & Therapeutics (Arnaud, 1987) (peer reviewed, retrieved 2026-04-28)
  2. Effects of caffeine on sleep taken 0, 3, or 6 hours before going to bed — J Clin Sleep Med (Drake et al., 2013) (peer reviewed, retrieved 2026-04-28)
  3. Caffeine intake during pregnancy — EFSA scientific opinion — EFSA Panel on Dietetic Products (2015) (guideline, retrieved 2026-04-28)