Caffeine Half Life Calculator
Caffeine Half Life Calculator
See how much caffeine is left in your body at bedtime from the drinks you had and when you had them, and when it falls below a level you choose. Also finds the latest time for a drink, checks a daily total against limits and tabulates 24 hours of decay.
Caffeine in your body through the day
Caffeine in the body = dose × 0.5 raised to (hours since the drink ÷ half life), added up over every drink. Gradual absorption uses an absorption half life of 6.8 minutes, so 99 percent is absorbed in 45 minutes. The half life is about 5 hours on average, with 1.5 to 9.5 hours reported. Limits are Health Canada 400 mg for adults, 300 mg in pregnancy, planning pregnancy or breastfeeding and 2.5 mg per kg for children, and ACOG 200 mg in pregnancy. Drink contents are Mayo Clinic averages. This is a planning estimate and general information, not medical advice. Exports contain only the values you entered.
How to use this tool
Choose what to calculate. "Caffeine left at bedtime" takes up to four drinks, each with its caffeine in milligrams and the clock time, plus your bedtime. It shows what is still in your body and when the total falls below a level you choose. "Latest time for a drink" works backwards from bedtime, and "Daily total against limits" compares your drinks with published limits. The last two modes look up a drink and show what remains of one dose.
The input people most often get wrong is the half life. The 5 hour preset is an average, not your number, and published values run from 1.5 to 9.5 hours. Try a short and a long value to see the spread. Enter caffeine in milligrams, not cup size, and use the label when there is one.
What the calculation is doing
Caffeine leaves the body by first order elimination. The body removes a fixed share of what is there each hour, not a fixed amount. The time for the amount to halve is the half life, so amount left = dose × 0.5 ^ (hours since the drink ÷ half life). With a 5 hour half life, 200 mg becomes 100 mg after 5 hours, 50 mg after 10 and 25 mg after 15.
Each drink is worked out on its own and the results are added, so a second coffee does not restart the clock on the first. For one drink the time to reach a level is hours = half life × log₂(dose ÷ level). A 200 mg drink reaches 25 mg after 3 half lives, because 200 ÷ 25 = 8 = 2³, and halving the dose saves exactly one half life.
Why the same coffee hits people differently
The half life is the number that moves most between people. The National Academies review puts the average at about 5 hours in healthy people and the range at 1.5 to 9.5 hours. Blanchard and Sawers measured 2.7 to 9.9 hours in 10 healthy men, more than a threefold spread in one small group.
Put that spread on one drink. A 200 mg coffee at 3 pm leaves 0.8 mg at 3 am for a 1.5 hour half life, 37.9 mg for 5 hours and 83.3 mg for 9.5 hours. One person is left with almost nothing and another still carries more than 40 percent of the cup.
Smoking shortens the half life, by up to 50 percent according to StatPearls. Oral contraceptives can double it (National Academies). Pregnancy lengthens it: Knutti and colleagues found 3 hours in women who were not pregnant and 10.5 hours in the last four weeks, and StatPearls gives up to 15 hours in the final trimester. Liver disease also lengthens it, but the sources checked give no single figure, so there is no preset for it.
The smoker preset of 3 hours is 40 percent below the average, inside the "up to 50 percent" reduction. The FDA notes wide variation in how fast people eliminate caffeine, and no calculator knows which end you are on. Read every answer as a planning estimate, uncertain by a factor of several either way.
How absorption changes the early curve but not the late tail
By default each drink is treated as absorbed at once, which is the simplest model and the one you can check on paper. Real absorption takes a little time: the National Academies report that 99 percent is absorbed within 45 minutes, with peak blood levels 15 to 120 minutes after drinking.
Gradual absorption uses an absorption half life of 6.8 minutes, which puts 99 percent in by 45 minutes. The peak comes 33 to 43 minutes after the drink for half lives of 3 to 9 hours. It is lower than the dose because elimination has already started: 183 mg for 200 mg at 5 hours.
The two models cross near the peak time. Before it gradual shows less in the body, after it slightly more, settling about 2.3 percent above the instant curve. It matters for a drink close to the moment you look: 200 mg at 10:30 pm with bed at 11 pm leaves 187 mg instant and 181 mg gradual. For a breakfast coffee it barely matters at bedtime.
What is left of a 200 mg drink, hour by hour
Instant absorption. A 100 mg drink leaves half of each figure and a 400 mg drink double. After 12 hours the 9 hour case still holds 79.4 mg, while the 3 hour case is down to 12.5 mg.
| Hours after the drink | 3 hour half life | 5 hour half life | 7 hour half life | 9 hour half life |
|---|---|---|---|---|
| 0 | 200 mg | 200 mg | 200 mg | 200 mg |
| 1 | 159 mg | 174 mg | 181 mg | 185 mg |
| 2 | 126 mg | 152 mg | 164 mg | 171 mg |
| 3 | 100 mg | 132 mg | 149 mg | 159 mg |
| 4 | 79.4 mg | 115 mg | 135 mg | 147 mg |
| 6 | 50 mg | 87.1 mg | 110 mg | 126 mg |
| 8 | 31.5 mg | 66 mg | 90.6 mg | 108 mg |
| 10 | 19.8 mg | 50 mg | 74.3 mg | 92.6 mg |
| 12 | 12.5 mg | 37.9 mg | 61 mg | 79.4 mg |
| 15 | 6.3 mg | 25 mg | 45.3 mg | 63 mg |
| 18 | 3.1 mg | 16.5 mg | 33.6 mg | 50 mg |
| 24 | 0.8 mg | 7.2 mg | 18.6 mg | 31.5 mg |
What common drinks contain, and how long one serving takes to clear
Drink contents are the Mayo Clinic averages of February 2025, the one source the calculator uses. The last column is the time for one serving to fall below 25 mg at a 5 hour half life. Health Canada lists 7 mg for an ounce of milk chocolate and 19 mg for sweet chocolate.
| Drink | Serving | Caffeine | Servings in 400 mg | Below 25 mg after, 5 hour half life |
|---|---|---|---|---|
| Brewed coffee | 8 oz | 96 mg | 4.2 | 9 h 42 min |
| Espresso | 1 oz | 63 mg | 6.3 | 6 h 40 min |
| Instant coffee | 8 oz | 62 mg | 6.5 | 6 h 33 min |
| Decaf brewed coffee | 8 oz | 1 mg | 400 | under 25 mg at the start |
| Black tea, brewed | 8 oz | 48 mg | 8.3 | 4 h 42 min |
| Green tea, brewed | 8 oz | 29 mg | 13.8 | 1 h 4 min |
| Cola | 8 oz | 33 mg | 12.1 | 2 h |
| Energy drink | 8 oz | 79 mg | 5.1 | 8 h 18 min |
| Energy shot | 2 oz | 200 mg | 2 | 15 h |
A worked example you can check on paper
Two 96 mg coffees, at 8 am and 1 pm, half life 5 hours, bed at 11 pm. The 8 am drink is 15 hours before bed, which is 15 ÷ 5 = 3 half lives, so 96 × 0.5³ = 96 × 0.125 = 12 mg. The 1 pm drink is 10 hours before bed, 2 half lives, so 96 × 0.25 = 24 mg. The total at bedtime is 12 + 24 = 36 mg.
The peak comes just after the 1 pm drink: 96 + 96 × 0.5^(5 ÷ 5) = 144 mg, and 36 ÷ 144 = 25%. To reach 25 mg the total must fall by a factor of 36 ÷ 25 = 1.44, which takes 5 × log₂(1.44) = 2.63 hours, so 1:38 am.
Latest time for one 96 mg coffee to leave 25 mg at 11 pm: 5 × log₂(96 ÷ 25) = 5 × 1.941 = 9.71 hours, or 9 h 42 min before bed, so 1:18 pm.
Mistakes that catch people out
Using the average half life as your own. Across the published range of 1.5 to 9.5 hours, one 200 mg coffee at 3 pm leaves from under 1 mg to 83.3 mg at 3 am. Try two or three half lives.
Entering the cup size instead of the milligrams. An 8 ounce brewed coffee is about 96 mg by Mayo Clinic and 135 mg by Health Canada, so use the label when you have one.
Leaving out the morning drink. A 96 mg coffee at 8 am is 15 hours before an 11 pm bedtime, so an eighth is left. That is 12 mg, a third of the 36 mg in the default example.
Reading the chosen level as a sleep threshold. The 25 mg level is a marker for comparing scenarios, not a value from research. One 200 mg drink at 5 hours takes 10 hours to reach 50 mg, 15 to reach 25 mg and 21.6 to reach 10 mg.
Bedtimes after midnight. A bedtime earlier than your last drink is read as the next day, so a 1 am bedtime with a 3 pm drink means 10 hours later. In the latest time mode, bedtimes before 6 am count as after midnight.
Popular calculations
Each opens the calculator with those figures filled in, and they can be changed once it is open.
Caffeine left at bedtime
Latest time for a drink
Daily total against limits
What is in a drink
What remains after a set time
How this was checked
The closed form was compared with a step by step simulation of caffeine moving from the gut into the body and out of it, in 40 random cases with one to four drinks, half lives from 1 to 15 hours and both absorption models. Across 12,000 samples the worst difference was under one part in ten billion of the dose. Round trips of the time to a level came back to within 10⁻¹³ mg.
Published values were reproduced. StatPearls gives 90, 94 and 97 percent eliminated after 3.3, 4 and 5 half lives, and the engine gives 89.85, 93.75 and 96.88 percent. The absorption rate is set from the National Academies figure of 99 percent absorbed in 45 minutes, so that one matches by construction. The peak time it produces, 38 minutes at a 5 hour half life, was not an input. It sits inside the 15 to 120 minute range and within one standard deviation of the 29.8 minute mean of Blanchard and Sawers.
Sources: National Academies (2001), StatPearls on caffeine and on drug half lives, Knutti and colleagues (1982), Blanchard and Sawers (1983), Health Canada (April 2025), the FDA (August 2024), ACOG Committee Opinion 462, Mayo Clinic (February 2025) and Drake and colleagues (2013). Every address listed above was opened and checked.
Common questions
What is the half life of caffeine?
It is the time the body takes to remove half of the caffeine in it. For healthy adults the mean is about 5 hours, with reported values from 1.5 to 9.5 hours (National Academies, 2001). Smoking, oral contraceptives and pregnancy all move it, so treat the presets as examples and type your own hours.
How long does caffeine stay in your system?
Caffeine never reaches zero, so the answer depends on the level you call gone. For a 200 mg drink and a 5 hour half life the body holds 100 mg after 5 hours, 25 mg after 15, 10 mg after 21.6 and 7.2 mg after 24. After 5 half lives about 97 percent has gone (StatPearls). With a 9 hour half life the same drink needs 27 hours to reach 25 mg.
When should I stop drinking coffee before bed?
The latest time mode answers a narrower question: given the drink, your half life and the level you want at bedtime, when must the drink be finished. For one 96 mg coffee, a 5 hour half life, bed at 11 pm and 25 mg, that is 1:18 pm. In a study of 12 healthy sleepers, 400 mg six hours before bed cut measured sleep by more than an hour, and they did not notice (Drake, 2013). At a 5 hour half life the tool puts that dose at 174 mg at bedtime, a modelled figure, since the study measured sleep and not blood levels. This tool gives general information and is not medical advice.
How much caffeine is too much in a day?
Published limits are daily totals. Health Canada gives 400 mg for adults, 300 mg for people who are pregnant, planning pregnancy or breastfeeding, and 2.5 mg per kg of body weight up to age 18. The FDA cites 400 mg for most adults, about two to three 12 ounce cups of coffee. ACOG says under 200 mg a day does not appear to be a major factor in miscarriage or preterm birth.
Do several drinks add up?
Yes. Each is worked out on its own and the results are added. Two 96 mg coffees at 8 am and 1 pm leave 12 mg and 24 mg at 11 pm, a total of 36 mg. Three 96 mg coffees at 8 am, noon and 4 pm leave 69.3 mg. The latest drink counts most because it has had the least time to decay.
How much caffeine is in a cup of coffee?
Sources differ, and real cups differ more. Mayo Clinic lists 96 mg for 8 ounces of brewed coffee, Health Canada 135 mg, and the FDA 113 to 247 mg for 12 ounces. The calculator uses the Mayo Clinic figures throughout so drinks compare fairly.
Related Calculators and Reference
- Sleep Calculator: a bedtime or wake time to bring back here.
- Half Life Calculator: the same decay law for any substance.
- Water Intake Calculator: a daily fluid target beside your coffee and tea.
- Pregnancy Weight Gain Calculator: the companion for a pregnancy, when the caffeine limits above are lower.
- BAC Calculator: estimate blood alcohol content from weight, drinks and time, with the limits of an estimate.
- Coffee Ratio Calculator: the coffee and water that a brew ratio gives.
- FDA: Spilling the Beans, How Much Caffeine Is Too Much?: daily amounts, drink contents and individual differences.