Will this coffee keep me up?

How the number is worked out

Half of what you drank is gone after about five hours, half of the rest five hours after that, and so on. In arithmetic that is the starting milligrams multiplied by a half, raised to the number of hours divided by five. A bedtime that reads earlier on the clock than the coffee is treated as the next day, so a drink at 16:00 and a bedtime at 01:00 is nine hours apart.

The drink figures are reference values by drink and brew method, not a measurement of your particular cup. Beans, dose, grind and extraction all move the real number, sometimes a long way.

Where the five hours is wrong

A single half-life is a simplification, and it is worth knowing how it fails. Between healthy adults the real figure ranges roughly three to seven hours, and genetics account for most of that spread. It runs considerably longer in pregnancy and for people on oral contraceptives, and shorter for smokers. Absorption is not instant either: blood levels peak thirty to sixty minutes after drinking, so the first half hour is overstated by any model that assumes it all arrives at once.

So every figure on this page is an estimate of an estimate. It is enough to see the shape of an evening, which is the useful part, and not enough to be a measurement of you.

The published reference points

These are population figures from EFSA and the FDA for healthy adults. They are not targets, and they are not advice for you specifically.

Every cup, not just this one

This page does one drink at a time. My Coffee Shelf estimates the caffeine in every brew you log, follows the running total through the evening, and shows what is left at bedtime without you typing anything. On iPhone it writes each brew to Apple Health, so tea and everything else counts too.