Caffeine half-life calculator
Enter the caffeine you had, how many hours ago, and a half-life. The calculator estimates how much is left now and how long until it drops to a level you pick, for example before bedtime. It is an estimate for planning, not a medical measurement.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Enter the caffeine in milligrams: check the label, or use the caffeine intake calculator for typical drinks.
- Enter the hours since you had it.
- Keep the 5-hour half-life or set your own.
- Optionally set a level, such as the amount you want left at bedtime.
Caffeine is cleared roughly exponentially:
left = dose × 0.5^(hours ÷ half-life)
and the time to reach a level is half-life × log₂(dose ÷ level). After one half-life half is left, after two a quarter, after three an eighth.
Worked example
200 mg of caffeine, 8 hours ago, half-life 5 hours:
- 8 ÷ 5 = 1.6 half-lives; 0.5^1.6 = 0.33;
- left: 200 × 0.33 = 66 mg (33 % of the dose), 134 mg already eliminated;
- to reach 50 mg: 5 × log₂(200 ÷ 50) = 5 × 2 = 10 hours after the drink.
Limits
- The model assumes all the caffeine has been absorbed; in reality absorption takes up to about an hour after drinking.
- Sensitivity differs as much as the half-life: the same amount keeps some people awake and not others.
- This is not medical advice. If you are pregnant, take medication or have a heart condition, ask a doctor about caffeine.
Frequently asked questions
What is the half-life of caffeine?
It is the time the body takes to clear half of the caffeine. In healthy adults it is typically somewhere around 3 to 7 hours, with 5 hours a common middle value, which is the default here. It varies a lot from person to person.
What makes the half-life longer or shorter?
Smoking tends to shorten it. Pregnancy, some oral contraceptives, some medicines and liver disease can lengthen it considerably, and genetics plays a part. If you know your caffeine lasts long, try 7 or 8 hours; if it wears off fast, try 3 or 4.
How much caffeine is left after 8 hours?
With a 5-hour half-life, 8 hours is 1.6 half-lives, and 0.5^1.6 = 0.33. A 200 mg coffee leaves about 66 mg, a third of the dose. With a 7-hour half-life it would be about 91 mg.
When should I stop drinking coffee before bed?
Use the level field: enter the amount you are comfortable with at bedtime and read the hours it takes to get there. From 200 mg to 50 mg takes two half-lives, 10 hours at a 5-hour half-life, so a 200 mg coffee at 2 pm is down to about 50 mg near midnight.
Can I add several drinks?
Each drink decays on its own, so you can work them out one by one and add what is left. For example, 100 mg 6 hours ago and 100 mg 2 hours ago at a 5-hour half-life leave about 44 + 76 = 120 mg.