MATH · FREE
Caffeine Half-Life Calculator
How much caffeine is left after a few hours? Start from a cited half-life of about 5 hours, change it to fit you, and see the amount remaining, a decay chart and a table.
Short answer
In healthy adults the average half-life of caffeine is approximately 4 to 5 hours (NIH-hosted reviews give 3 to 5 hours). Half of the caffeine is left after one half-life, a quarter after two. It varies from person to person.
Math only. Not medical advice. Caffeine half-life varies between people and changes with pregnancy, medicines and other factors.
AMOUNT REMAINING
Amount left
Percent left
Half-lives elapsed
| Half-lives | Time | Amount left | Percent left |
|---|
Math only. Not medical advice. Caffeine half-life varies between people and changes with pregnancy, medicines and other factors.
What the caffeine half-life means
The half-life is the time it takes for the amount of caffeine in your body to fall by half. Take 200 mg at 8:00 with a 5 hour half-life: about 100 mg remains at 13:00, 50 mg at 18:00 and 25 mg at 23:00. The formula is N = N0 × (1/2)t ÷ 5 with t in hours.
Same drink, three different half-lives
The cited range matters more than it looks. Starting from 200 mg, the amount left 10 hours later is:
- 3 hour half-life: 200 × (1/2)10/3 ≈ 19.8 mg.
- 5 hour half-life: 200 × (1/2)10/5 = 50 mg.
- 8 hour half-life: 200 × (1/2)10/8 ≈ 84.1 mg.
A longer half-life, as in late pregnancy, leaves much more behind. Type each value into the calculator above to see the curve.
How long until it is almost gone?
After 5 half-lives about 3% is left and after 7 less than 1%. With a 5 hour half-life that is 25 and 35 hours. To find the time to reach any level, switch the calculator to "Time to reach an amount", for example 200 mg down to 10% takes 5 × log2(10) ≈ 16.6 hours.
This page does one-drink arithmetic. It does not know your body, medicines or health, and it says nothing about how much caffeine is right for you. Talk to a clinician about your own limits.
Sources
- Struniewicz et al., Pregnancy and Caffeine Metabolism, Nutrients 2025 (PMC12526311) (accessed October 2026). States an average caffeine half-life of approximately 4 to 5 h in healthy adults, variation between individuals, and a half-life up to approximately 15 h in late pregnancy.
- Song et al., Caffeine: A Multifunctional Efficacious Molecule, Int. J. Mol. Sci. 2024 (PMC11593559) (accessed October 2026). Gives a caffeine half-life of 3 to 5 h.
Frequently asked questions
What is the half-life of caffeine?
A 2025 review in Nutrients states that in healthy adults the average half-life of caffeine is approximately 4 to 5 hours, and a 2024 review in the International Journal of Molecular Sciences gives 3 to 5 hours. This page starts from 5 hours, the top of that range, so the example leaves slightly more caffeine than a 4 hour value would. Change it to match your own estimate.
How long does caffeine last in your system?
It depends on the half-life you pick. With a 5 hour half-life, half is left after 5 hours, a quarter after 10, an eighth after 15 and about 3% after 25 hours. With a 3 hour half-life the same 3% mark comes at 15 hours. The effects you feel can fade sooner than the last traces leave the body.
Does pregnancy change the half-life of caffeine?
Yes. The 2025 Nutrients review says CYP1A2 activity decreases during pregnancy, which prolongs the half-life, and that it can extend up to approximately 15 hours in the late gestational trimester. If you are pregnant, set a longer half-life here and ask your clinician about caffeine.
Can I use this to time my last coffee before bed?
You can see how much of a starting amount is left after a number of hours, which is the same arithmetic as the half-life formula. For a version that adds several drinks and shows the amount at your bedtime, use the caffeine calculator linked below.
Why does the amount never reach zero?
Each half-life removes half of whatever is left, so the amount keeps shrinking by half without hitting exactly zero. After 10 half-lives about 0.1% remains, which is why the table stops at 6 half-lives.
Math only. Not medical advice. Caffeine half-life varies between people and changes with pregnancy, medicines and other factors.
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