How much caffeine is left in your body from up to three doses, based on its half-life, and when the level drops below an amount you choose.
By AdminDrafted with AI writing toolsUpdated Checked examples: 5
At 22:00 about 45 mg of the 221 mg you had is still in your body. With a 5-hour half-life it drops below 50 mg at 21:15, 7 h 14 min after the last dose.
Times are hours after the first dose at 08:00; doses after the check time are left out.
Rounded up to the next whole minute.
Each dose is assumed to enter the body at once and then halve every half-life: the amount left from a dose is dose × 0.5^(hours since the dose ÷ half-life), and the calculator adds up to three doses. It also works out when the total falls below a level you choose, using time = half-life × log2(level after the last dose ÷ chosen level).
People use it to decide when to stop drinking coffee before bed. With the defaults, 150 mg at 08:00 and a 71 mg black tea at 14:00 leave about 45 mg at 22:00, and the level has been below 50 mg since 21:15.
The default half-life of 5 hours is the average for healthy adults, but it ranges from about 1.5 to 9.5 hours and changes with pregnancy, oral contraceptives and smoking. Blood levels actually peak 15 to 120 minutes after drinking, so the curve overstates the level in the first hour or two after each dose.
With the average 5-hour half-life, half of a dose is left after 5 hours, a quarter after 10 and an eighth after 15. The amount left is dose × 0.5^(hours ÷ half-life), so 200 mg at 08:00 leaves 50 mg at 18:00. In healthy adults the half-life ranges from about 1.5 to 9.5 hours.
For one dose, raise 0.5 to the power of hours elapsed ÷ half-life and multiply by the dose. 200 mg after 10 hours at a 5-hour half-life is 200 × 0.5² = 50 mg; after 25 hours, 0.5⁵ leaves about 3%, or 6 mg.
For several doses, work out each one from its own time and add them. 150 mg at 08:00 and a 71 mg black tea at 14:00, checked at 22:00:
The later dose has had less time to fade, which is why an afternoon tea can matter more at bedtime than a larger morning coffee.
Solve the decay for time: hours = half-life × log2(amount now ÷ target level). Just after the 14:00 tea the total is 136.3 mg, so falling to 50 mg takes 5 × log2(136.3 ÷ 50) = 7.23 hours, until just after 21:14.
Doses that cross midnight work the same way. 150 mg at 20:00 still leaves 65 mg at 02:00 and falls below 50 mg at 03:55. The 50 mg level is your own choice, not a medically validated point below which caffeine stops affecting sleep.
The FDA gives these amounts for 12 fl oz servings:
An 8 fl oz decaf coffee has 2–15 mg. Brewing method and cup size change the dose, so use the label figure where there is one.
The 5-hour figure is an average. Half-lives in healthy adults run from about 1.5 to 9.5 hours (Antonio et al., 2024), longer in pregnancy and with oral contraceptives and shorter in smokers (Institute of Medicine).
One study measured a mean of 8.3 hours in pregnant women against 3.4 hours in other adults (Knutti et al., 1981). At 8.3 hours, the morning coffee and afternoon tea above leave 83 mg at 22:00 and stay above 50 mg until 04:05 the next morning.
The decay model also assumes each dose enters the body at once. Blood levels actually peak 15 to 120 minutes after drinking, so the estimate runs high in the first hour or two after each dose.
A 2023 meta-analysis of 24 studies (Gardiner et al.) puts the cut-off at least 8.8 hours before bed for a 107 mg cup of coffee and 13.2 hours for a 217.5 mg pre-workout drink.
Across those studies, caffeine cut total sleep by 45 minutes and added 9 minutes to the time taken to fall asleep. For a 23:00 bedtime, 8.8 hours means finishing that cup of coffee by 14:12.
For most healthy adults, the FDA cites 400 mg a day, about two to three 12 fl oz cups of brewed coffee, as an amount not generally associated with negative effects.
The FDA estimates that toxic effects such as seizures can appear after rapidly consuming about 1,200 mg.
During pregnancy, the reference figure is 200 mg a day. The American College of Obstetricians and Gynecologists' Committee Opinion 462 (reaffirmed 2026) states that moderate intake, less than 200 mg a day, does not appear to be a major contributing factor in miscarriage or preterm birth.
All three use a 5-hour half-life and a 50 mg target level.
| Scenario | Remaining at check (mg) | Threshold clock time | Entered total (mg) |
|---|---|---|---|
| Default | 45.0 | 21:15 | 221 |
| Two half-lives | 50.0 | 18:00 | 200 |
| Overnight dose | 65.3 | 03:56 (+1 day) | 150 |
Threshold times are rounded up to the next minute: the overnight crossing falls at 03:55:29.
The sleep cycle calculator finds a bedtime from your alarm in 90-minute cycles. The blood alcohol calculator uses a different, straight-line elimination model for alcohol.
Checked against: 200 × 0.5^(10/5) = 50; threshold reached at 5 × log2(200/50) = 10 h
Checked against: 95·0.5^(14/5) + 64·0.5^(9/5) = 32.01997; 13:00 + 5·log2(111.5/50) h = 18:47:08, rounded up to 18:48
Checked against: 150 × 0.5^(6/5) = 65.2913; 5·log2(3) = 7.9248 h after 20:00 = 03:55:29, rounded up to 03:56
Checked against: 40 × 0.5^(3/5) = 26.3902; 40 mg ≤ 50 mg from the start
With the average 5-hour half-life, half of a dose is still in your body after 5 hours, a quarter after 10 and about 3% after 25, so 200 mg of coffee at 08:00 leaves 50 mg at 18:00. Half-lives in healthy adults range from about 1.5 to 9.5 hours (Antonio et al., Journal of the International Society of Sports Nutrition, 2024), so the same cup can last several times longer in one person than in another.
For most healthy adults the FDA cites 400 mg a day, about two to three 12 fl oz cups of brewed coffee, as an amount not generally associated with negative effects. The FDA estimates that toxic effects such as seizures can appear after rapidly consuming about 1,200 mg. The calculator warns when the doses you enter add up to more than 400 mg.
At least 8.8 hours for a 107 mg cup of coffee and 13.2 hours for a 217.5 mg pre-workout drink, according to a 2023 meta-analysis of 24 studies in Sleep Medicine Reviews (Gardiner et al.). Across those studies caffeine cut total sleep by 45 minutes and added 9 minutes to the time taken to fall asleep. In an earlier trial, 400 mg taken 6 hours before bed still disrupted sleep (Drake et al., 2013).
The FDA gives 113 to 247 mg for 12 fl oz of brewed coffee, 71 mg for black tea, 37 mg for green tea, 23 to 83 mg for soft drinks and 41 to 246 mg for energy drinks of the same size; 8 fl oz of decaf has 2 to 15 mg. Brewing method and cup size change the dose, so use the label figure where there is one.
Less than 200 mg a day. The American College of Obstetricians and Gynecologists' Committee Opinion 462 states that intake below 200 mg a day does not appear to be a major contributing factor in miscarriage or preterm birth. Caffeine is also cleared more slowly: one study measured a mean half-life of 8.3 hours in pregnant women against 3.4 hours in other adults (Knutti et al., 1981), so a longer half-life gives a more realistic curve.
For information only, not medical advice. These formulas describe populations, not you; talk to a clinician before acting on a result.
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