Caffeine Half-Life Calculator

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.