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Caffeine Half-Life Calculator

See how much caffeine is left in your body before bedtime.

Result

66 mg
33.0%
134 mg

How it is calculated

The tool applies exponential decay: remaining = dose × 0.5(hours ÷ half-life), with the dose in mg and both times in hours. The percentage is remaining ÷ dose × 100 to one decimal, and eliminated is dose − remaining. The half-life is an input you must provide, there is no default, so the answer is only as good as the value you choose. Zero elapsed hours is allowed and returns 100%, but a negative time, a zero dose or a zero half-life gives no result. The tool treats one dose only: it does not add up several cups drunk at different times, and it models the amount in the body, not how alert you feel.

Worked example

You drink 200 mg of caffeine and want to know the amount left 8 hours later, using a half-life of 5 hours. The exponent is 8 ÷ 5 = 1.6, and 0.5 raised to 1.6 is about 0.330. Remaining = 200 × 0.330 = 66 mg, which is 33.0% of the dose. Eliminated = 200 − 66 = 134 mg. With a slower 8-hour half-life the same 8 hours would leave exactly half, 100 mg.

Try this example →

Common mistakes

  • Entering the half-life in minutes or the elapsed time in minutes. Both fields are hours, so 90 minutes is 1.5.
  • Using the same half-life for everyone. Individuals vary a lot, and a value that is too short makes the leftover look smaller than it is, which is the risky direction for evening coffee.
  • Entering the cup size in ml instead of the caffeine in mg. The dose box is milligrams of caffeine, and the amount per drink depends on the drink.
  • Expecting the total of several drinks. The tool decays a single dose, so for two coffees you must work out each separately, or add the results by hand.
  • Reading 0 mg remaining as the moment caffeine stops affecting you. The curve never reaches zero, and the number shown is rounded to a whole mg.

Sources

About this calculator

Caffeine is metabolized by the liver enzyme CYP1A2, and in a typical healthy adult it follows first-order elimination kinetics with an average biological half-life of about five hours — meaning roughly half the caffeine you consumed is still circulating five hours later, and a quarter remains at ten hours. This average masks huge individual variation: genetic differences in CYP1A2 activity, pregnancy, oral contraceptive use, liver function, and smoking status can shrink the half-life to two to three hours in fast metabolizers or stretch it past nine hours in slow ones.

Sleep researchers and coaches use half-life estimates like this to explain why an afternoon coffee can still disrupt sleep onset at bedtime, and why athletes time caffeine intake around 30 to 60 minutes before a workout to catch blood levels near their peak. Because caffeine also has a stimulant effect on the central nervous system long after most of it has cleared, the half-life curve is a rough guide to residual stimulant exposure, not a precise predictor of alertness.

This calculator applies the standard first-order decay formula to your dose and elapsed time, so you can see approximately how many milligrams of caffeine are still in your system at any point after your last cup.

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