Caffeine Half-Life Calculator

Caffeine has an average half-life of around 5 hours in healthy adults, so half the dose is still circulating five hours later. This calculator lets you pick a common drink (or enter a custom mg), say when you drank it, and see the decay curve plus the time until you cross the ~100 mg "shouldn't affect sleep" threshold. Half-life is tunable — pregnancy can double it, smokers metabolize it faster.

The curve is exponential decay at a constant half-life, which is a reasonable model for ordinary doses and a simplification in two places. Absorption is not instantaneous: blood levels peak roughly 30 to 60 minutes after drinking, so the first stretch of the curve overstates how much is actually circulating. And the five-hour figure is a population average concealing a wide spread. Liver enzyme activity varies several-fold between individuals, oral contraceptives roughly double the half-life, pregnancy can stretch it past 15 hours in the third trimester, and heavy smoking cuts it to about half. Adjust the half-life if you know yours runs long or short.

The 100 mg threshold is a rule of thumb rather than a clinical standard, and sensitivity to a given blood level differs enormously between people. Some sleep through an espresso at midnight and others notice a morning coffee at bedtime, which no calculator can predict for you.

The drink figures are typical values, not measurements of what you drank. Brewed coffee varies with bean, grind and brew time; a large chain coffee can carry two or three times what a home cup does; and energy drinks and pre-workout supplements often list caffeine per serving where the container holds more than one. Where a label gives an exact number, enter it as a custom dose.

This is arithmetic on an average, offered for curiosity and rough planning. It is not medical advice, and anything involving a heart condition, a pregnancy or a medication interaction belongs with a doctor rather than a decay curve.