Caffeine Half-Life Calculator

Caffeine leaves the body in a way that is easy to model: it decays exponentially, meaning the amount left halves at a steady interval. That interval, the half-life, averages about five hours in healthy adults, but ranges from about three hours in fast metabolisers to seven or more in slow ones. The difference is largely genetic, and it is controlled by the same liver enzyme, CYP1A2, that also handles a number of medications.

The practical consequence is sobering. A 150-milligram coffee at three in the afternoon — a single large filter coffee or a double espresso — leaves about 75 milligrams circulating at eight in the evening, 37 at one in the morning, and still a measurable amount when the alarm goes off. That is why caffeine disrupts sleep even when you feel perfectly capable of falling asleep: it reduces the amount of slow-wave sleep, shortens total sleep time and increases the number of times you wake, often without your noticing.

The usual coffee figures are worth carrying in your head. A single espresso is around 60 to 80 milligrams. A cup of filter coffee is 90 to 150. A strong large coffee can exceed 200. A cup of black tea is about 50, green tea 30, a cola 20 to 35, and an energy drink 80 to 160. Most pre-workout powders and supplements are far higher than people expect — often 200 to 350 milligrams per scoop.

There is no universal cut-off, but the evidence points to a widely-quoted rule of thumb: stop caffeine about eight to ten hours before bed if you want it to have minimal effect on sleep. If you are a slow metaboliser — or take oral contraceptives, which slow clearance substantially — the sensible window is longer. Pregnant women are generally advised to keep total intake under 200 milligrams a day. If you have palpitations, anxiety, high blood pressure or a heart condition, or if you take medications that interact with caffeine, ask a doctor rather than relying on a calculator. Caffeine is a drug with real effects, and dose and timing are the two things you actually control.

This calculator returns estimates for general information only. It is not medical advice, diagnosis or treatment, and it cannot replace an assessment by a qualified health professional. Individual results vary, and figures such as calorie needs, body-fat percentage and heart-rate zones are approximations based on population averages rather than measurements of you.