Caffeine Half-Life Calculator
Track caffeine levels in your system over time. Pick a source, time of intake + bedtime — see decay curve, bedtime levels, and "caffeine cutoff" for sleep.
Caffeine Half-Life Calculator
Energy drinks, cans of cola and coffee-shop drinks vary too much for one number: the US FDA gives 41 to 246 mg for 12 fl oz of energy drink. Pick Custom amount and type the figure on the label.
Enter what you drank and when, and your bedtime.
How to use the Caffeine Half-Life Calculator
Enter each drink and when you had it
Pick the closest drink or type the amount from the label. The listed amounts are typical values from the US Department of Agriculture's FoodData Central, such as 95 mg for an 8 fl oz cup of brewed coffee and 63 mg for a single espresso shot. Real cups vary a lot: the FDA gives 113 to 247 mg for a 12 fl oz cup of regular brewed coffee. Use Add another drink for each cup in the day, up to six.
Set your bedtime
Times are on your own clock. A drink counts as taken in the 24 hours before bedtime, so with a bedtime of 01:00 a coffee at 22:00 has had three hours to clear. The timeline runs hour by hour from your first drink to bedtime.
Adjust the half-life if you have a reason to
The default is 5 hours, a round figure inside what studies of healthy adults have measured. Measured averages run longer in pregnancy (8.3 hours, ranging up to 16) and on oral contraceptives (7.9 hours), and shorter in smokers (3.5 hours). The field accepts 1 to 24 hours.
Read the bedtime figure and the cutoff
The threshold starts at 100 mg because EFSA found that a single dose of that size may affect sleep in some adults when taken close to bedtime. It is a starting point, not a measured line, so set your own. The cutoff tells you the latest time for your last drink to leave you under that threshold, allowing for everything you drank before it.
Caffeine half-life — why your afternoon coffee is still in your system at midnight
Caffeine's half-life in healthy adults is a few hours, and it varies a lot between people. One study measured a mean of 3.4 hours (range 2 to 5) in men and non-pregnant women; others found 5.4 hours in women not taking oral contraceptives and 6.0 hours in non-smokers. This calculator defaults to 5 hours, a round figure inside that spread; change it if you know yours is different. On that default, 100 mg of caffeine at noon leaves 50 mg at 17:00, 25 mg at 22:00 and 12.5 mg at 03:00.
That tail matters for sleep. A 2023 review that pooled 24 studies found caffeine cut total sleep by 45 minutes on average, lowered sleep efficiency by 7% and cut deep sleep by 11.4 minutes (Gardiner et al., Sleep Medicine Reviews, 2023). In a home study, 400 mg taken 6 hours before bed still disturbed sleep compared with a placebo (Drake et al., 2013). On this calculator's 5-hour default, 400 mg taken 6 hours before bed would leave about 174 mg at bedtime.
How the half-life maths works
Caffeine is cleared by first-order elimination: the amount removed each hour is a fixed share of what is left, so the amount halves every half-life. After one half-life 50% remains, after two 25%, after three 12.5%, after four 6.25%. On a 5-hour half-life, 200 mg at noon is 100 mg at 17:00, 50 mg at 22:00, 25 mg at 03:00 and 12.5 mg at 08:00 the next morning. A second drink taken before the first has cleared adds to what is left, which is why the calculator takes every drink and adds them up.
The calculator treats each drink as absorbed the moment you finish it, which overstates the first hour. In one study, caffeine reached its peak in the blood 0.79 hours after a dose in women not on oral contraceptives and 1.52 hours after it in women on them (Abernethy and Todd, 1985). By bedtime the difference is small.
"To avoid reductions in total sleep time, coffee (107 mg per 250 mL) should be consumed at least 8.8 h prior to bedtime."
Gardiner et al., The effect of caffeine on subsequent sleep: A systematic review and meta-analysis, Sleep Medicine Reviews (2023)
CYP1A2 genetics and individual variation
Most caffeine is cleared by the liver enzyme CYP1A2, and a common variant in its gene sorts people into faster and slower metabolisers. In one large study (Cornelis et al., JAMA, 2006) people homozygous for CYP1A2*1A were "rapid" metabolisers and carriers of *1F were "slow", and more than half the participants carried the slow allele. Genotype is only part of the picture, and other measured effects are larger. Pregnancy: a mean half-life of 8.3 hours (range 3 to 16) against 3.4 hours in non-pregnant adults, returning to normal within about a month of delivery in most women (Knutti et al., 1981). Oral contraceptives: 7.9 hours against 5.4 hours in matched controls (Abernethy and Todd, 1985). Smoking speeds clearance: 3.5 hours in smokers against 6.0 hours in non-smokers (Parsons and Neims, 1978).
Coffee, tea and kopi in ASEAN
Kopi in Singapore and Malaysia, Vietnamese cà phê, Indonesian kopi tubruk and Thai iced tea are made to recipes that vary from stall to stall, and we found no published laboratory measurement of their caffeine that we could check. Strength depends on the beans, the amount used and how long it brews, so treat any single figure for them with caution and use Custom amount if you know yours. What the calculator can show is the total. Four ordinary cups of brewed coffee at 95 mg each come to 380 mg, close to the 400 mg a day that the FDA says is not generally associated with negative effects in most adults. A day of kopi and teh adds up the same way.
How long caffeine stays in you, and what changes it
Half-life in healthy adults: a few hours. A mean of 3.4 h (range 2 to 5) in one study, 5.4 h and 6.0 h in the control groups of two others. This tool defaults to 5 h.
On a 5-hour half-life, 200 mg at noon is still 50 mg at 22:00: two half-lives leave a quarter.
CYP1A2 does most of the work. A common variant in its gene separates rapid from slow metabolisers; in a JAMA study of over 4,000 people, more than half carried the slow allele.
Pregnancy extends half-life to a mean of 8.3 hours (range 3 to 16). EFSA found up to 200 mg a day raises no safety concerns for the foetus.
400 mg a day is the amount the FDA says is not generally associated with negative effects in most adults, about two to three 12 fl oz cups of coffee.
In a home study, 400 mg taken 6 hours before bed still disturbed sleep compared with a placebo.
EFSA found a single 100 mg dose may affect sleep duration and patterns in some adults when taken close to bedtime.
Decaf is not caffeine-free: the FDA says decaf coffee typically has 2 to 15 mg in an 8 fl oz cup.
In the pregnancy study, individual half-lives could not be correlated with age, weight or how much coffee people drank.
A 2023 review advises coffee at least 8.8 hours before bed, and a pre-workout serve at least 13.2 hours. For 23:00 that is 14:12 and 09:48.
Frequently Asked Questions
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Maybe, but feeling fine is not the same as sleeping the same. In a home study, 400 mg taken 6 hours before bed still disturbed sleep compared with a placebo when it was measured with a sleep monitor (Drake et al., 2013), and a 2023 review of 24 studies found caffeine cut total sleep by 45 minutes on average (Gardiner et al., 2023). If you want to know, stop afternoon caffeine for a couple of weeks and keep a simple sleep diary.
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For most healthy adults, the FDA cites 400 mg a day (about two to three 12 fl oz cups of coffee) as an amount not generally associated with negative effects, and EFSA reaches the same figure. EFSA also found single doses up to 200 mg of no concern for healthy adults, up to 200 mg a day of no concern for the foetus in pregnancy, and 3 mg per kg of body weight a day for children and adolescents. The FDA advises talking to a health care provider if you are pregnant, trying to become pregnant or breastfeeding. Signs of too much caffeine it lists include a fast heart rate, palpitations, insomnia, anxiety, jitters, upset stomach, nausea and headache, and it estimates that rapid consumption of around 1,200 mg can cause toxic effects such as seizures. If you have a heart condition or take regular medicines, ask your doctor what is right for you.
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Only a measurement gives it in hours: the studies on this page took repeated saliva or blood samples after a known dose. A genetic test can report your CYP1A2 variant, which sorts people into faster and slower metabolisers, but it does not give a half-life, and pregnancy, smoking, oral contraceptives and some medicines move it more. How an evening coffee affects your sleep is a rough guide to sensitivity, but it mixes metabolism with tolerance and sleep habits. If you have no better figure, keep the default and adjust it for the factors above.
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Yes. In one study the mean half-life was 3.4 hours in non-pregnant adults and 8.3 hours in pregnant women (range 3 to 16 hours), and in most women it returned to normal within a month of delivery (Knutti et al., 1981). On those averages the same coffee takes about two and a half times as long to clear. EFSA found intakes up to 200 mg a day raise no safety concerns for the foetus; caffeine also passes into breast milk, so ask your doctor or midwife about intake while breastfeeding.
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It can. The FDA lists anxiety, jitters, a fast heart rate and palpitations among the signs of too much caffeine, and notes wide variation in how sensitive people are. If you notice anxiety after coffee, try cutting back gradually and see whether it changes; the FDA says withdrawal is not considered dangerous but can be unpleasant. If anxiety is a regular problem, talk to your doctor.
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No. The FDA says decaf coffee typically has 2 to 15 mg in an 8 fl oz cup. A laboratory study of coffee-shop decaf found 0 to 13.9 mg in a 16 oz serving, and 3.0 to 15.8 mg in a single decaf espresso shot from one outlet (McCusker et al., 2006). For most people that is small, but it is not zero, and the calculator will take it as a Custom amount.
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Within an hour or two. After an overnight fast, caffeine reached its peak in the blood 0.79 hours after a dose in one group of healthy women and 1.52 hours after it in women on oral contraceptives (Abernethy and Todd, 1985). This calculator treats each drink as absorbed at once, so its first hour is slightly high; the half-life itself is a separate measure, of how fast the body clears caffeine once it is in the blood.
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Not in the study that looked. Knutti and colleagues found individual half-lives could not be correlated with how much coffee people drank, or with age or weight (1981). Regular drinkers can feel less of the effect, but the caffeine is still there. If you want to cut back, the FDA suggests doing it gradually.
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No. The calculation runs in your browser, and the drinks, times and settings you enter are not sent anywhere.
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The Sleep Calculator counts back from a wake time in 90-minute cycles, and the BAC Calculator estimates blood alcohol, another common reason for broken sleep.
Method & sources
How it computes
CaffeineKernel: remaining = dose × 0.5^(hours ÷ half-life) for each drink, timed in the 24 h before bedtime, the drinks summed; absorption treated as instant. Half-life 5 h by default (inside measured adult values), adjustable 1-24 h. Cutoff for the last drink = half-life × log2(its dose ÷ room left under the threshold). Threshold 100 mg by default (EFSA single-dose sleep finding). Daily total against FDA/EFSA 400 mg and EFSA 200 mg in pregnancy.
What this tool implements
- First-order elimination, C(t) = C0 × 0.5^(t ÷ half-life); several drinks add, each decaying from its own time. The page says it treats each drink as absorbed at once, which overstates the first hour: Abernethy & Todd measured peak concentration 0.79 h after a fasted dose in controls and 1.52 h on oral contraceptives.
- Half-life values are the measured ones: Knutti 1981 mean 3.4 h (range 2-5, n = 25) in men and non-pregnant women and 8.3 h (range 3-16, n = 57) in pregnancy, normal within a month of delivery in most cases, and not correlated with age, weight or coffee consumption; Abernethy & Todd 1985 7.88 h on low-dose oral contraceptives vs 5.37 h in controls; Parsons & Neims 1978 3.5 h in smokers vs 6.0 h in non-smokers. Cornelis 2006: *1A/*1A rapid, *1F carriers slow, 54-55% carried *1F.
- EFSA 2015 Scientific Opinion on the safety of caffeine: single doses up to 200 mg and 400 mg/day of no concern for healthy adults, 200 mg/day for the foetus in pregnancy, 3 mg/kg bw/day for children and adolescents, and single doses of 100 mg "may affect sleep duration and patterns in some adults, particularly when consumed close to bedtime" — the source of the page's default 100 mg threshold, which the page calls a starting point, not a measured line.
- US FDA consumer update (content current as of 28 Aug 2024; read from the Internet Archive copy of 8 Oct 2026, the live page refusing automated requests): 400 mg/day not generally associated with negative effects, about two to three 12 fl oz cups; 113-247 mg per 12 fl oz of regular brewed coffee; energy drinks 41-246 mg per 12 fl oz; decaf typically 2-15 mg per 8 fl oz; toxic effects such as seizures around 1,200 mg consumed rapidly.
- Sleep: Drake 2013, 400 mg at 0, 3 or 6 h before bed each disturbed sleep vs placebo; Gardiner 2023 (24 studies) total sleep -45 min, efficiency -7%, deep sleep -11.4 min, coffee (107 mg per 250 mL) at least 8.8 h and a 217.5 mg pre-workout serve at least 13.2 h before bedtime.
- Preset amounts are USDA FoodData Central SR Legacy (mg per 100 g × 237 g for 8 fl oz, 29.6 g for 1 fl oz): brewed coffee 40 → 95 mg, espresso 212 → 63 mg per shot, instant 26 → 62 mg, black tea 20 → 47 mg, green tea 12 → 28 mg; pre-workout is Gardiner 2023's 217.5 mg standard serve. Energy drinks, colas and coffee-shop drinks have no preset; the page sends readers to the label.
Sources
- Knutti R, Rothweiler H, Schlatter C. Effect of pregnancy on the pharmacokinetics of caffeine. Eur J Clin Pharmacol 1981;21(2):121-6. https://doi.org/10.1007/BF00637512
- Abernethy DR, Todd EL. Impairment of caffeine clearance by chronic use of low-dose oestrogen-containing oral contraceptives. Eur J Clin Pharmacol 1985;28(4):425-8. https://doi.org/10.1007/BF00544361
- Parsons WD, Neims AH. Effect of smoking on caffeine clearance. Clin Pharmacol Ther 1978;24(1):40-5. https://doi.org/10.1002/cpt197824140
- Cornelis MC, El-Sohemy A, Kabagambe EK, Campos H. Coffee, CYP1A2 genotype, and risk of myocardial infarction. JAMA 2006;295(10):1135-41. https://doi.org/10.1001/jama.295.10.1135
- Drake C, Roehrs T, Shambroom J, Roth T. Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. J Clin Sleep Med 2013;9(11):1195-200. https://doi.org/10.5664/jcsm.3170
- Gardiner C, Weakley J, Burke LM, et al. The effect of caffeine on subsequent sleep: a systematic review and meta-analysis. Sleep Med Rev 2023;69:101764. https://doi.org/10.1016/j.smrv.2023.101764
- McCusker RR, Fuehrlein B, Goldberger BA, Gold MS, Cone EJ. Caffeine content of decaffeinated coffee. J Anal Toxicol 2006;30(8):611-3. https://doi.org/10.1093/jat/30.8.611
- EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific Opinion on the safety of caffeine. EFSA Journal 2015;13(5):4102. https://doi.org/10.2903/j.efsa.2015.4102 (conclusions as summarised on https://www.efsa.europa.eu/en/topics/topic/caffeine)
What can make this go out of date
- None at runtime: the calculation runs in the reader's browser and nothing entered is sent anywhere.
Abridged — the full review record for this tool runs longer than the list above.
Pick up where you left off
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