Caffeine Half-Life Calculator
Caffeine has an average half-life of about 5 hours, which means half the caffeine you drink is still in your body 5 hours later. Use this tool to find out exactly when your last cup will drop below the sleep-safe threshold of 50 mg — and whether you'll be safe to sleep at your usual bedtime.
Smokers metabolize faster (~3 hrs); pregnancy and oral contraceptives slow it (~7–10 hrs).
Below the sleep-safe threshold (50 mg) by bedtime.
Caffeine decay over the next 24 hours
The shaded area shows how much caffeine remains in your body hour by hour. The dashed line marks the sleep-safe threshold of 50 mg.
How caffeine half-life works
Caffeine is metabolized by the liver, primarily by an enzyme called CYP1A2. Like many drugs, caffeine is eliminated from the body in a "first-order" pattern — meaning a constant fraction is removed per unit of time. The half-life is the time it takes for the concentration in your blood to drop by 50%. So if you start with 200 mg of caffeine in your system, after one half-life (5 hours) you'll have 100 mg left. After two half-lives (10 hours), 50 mg. After three (15 hours), 25 mg.
This calculator applies that exact formula. Enter your dose (or pick a beverage from the database to auto-fill it), set your intake time and bedtime, and the chart will show you the projected caffeine level hour by hour across the next 24 hours. The dashed line marks the 50 mg sleep-safe threshold — when the curve drops below that line, you should be able to fall asleep without caffeine-related disruption.
What changes caffeine half-life?
Your personal half-life can be very different from the 5-hour average. The biggest factors:
- Genetics — variations in the CYP1A2 gene split people into "fast" (~3 hour half-life) and "slow" (~7 hour half-life) metabolizers. About 50% of the population carries the slow variant.
- Smoking — smokers metabolize caffeine about 50% faster (half-life of ~3 hours), because smoking induces CYP1A2. Quitting smoking roughly doubles caffeine half-life overnight.
- Pregnancy — half-life stretches to 11 hours in the third trimester, which is why pregnant women are advised to cap intake at 200 mg/day.
- Oral contraceptives — estrogen slows CYP1A2, extending half-life to 7–10 hours in women on the pill.
- Age — newborns metabolize caffeine very slowly (half-life up to 80 hours), but by 6 months the half-life is close to the adult value.
- Liver disease — severe cirrhosis can extend half-life to 50+ hours.
If any of these factors apply to you, adjust the half-life input in the calculator to get a more accurate projection of your bedtime caffeine level.
The 8-hour rule for sleep
Most sleep researchers recommend stopping caffeine at least 8 hours before bedtime. That gives your body time to metabolize roughly three half-lives (about 87% elimination), bringing a 200 mg afternoon coffee down to about 25 mg at bedtime. If your bedtime is 10:30 PM, that means your last caffeine should be no later than 2:30 PM. Use the calculator above to find your personal cutoff — and pair it with the daily intake calculator to make sure your total for the day is also under the safe limit.