Caffeine Metabolism Calculator: How Much Is Left at Bedtime

Most caffeine calculators take one drink and ask how long ago you had it. Real days are not like that. A coffee at 8, another at 11 and a cola at 4 overlap, and what matters for sleep is how much of all three is still in you when the light goes off. This page adds them up. For a single dose and a half-life you type yourself, the caffeine half-life calculator does that one job.

How it adds up

Caffeine follows first-order elimination: every half-life, the amount left halves, whatever the amount was. Each drink decays on its own clock, and the level at any moment is the sum of what is left of each.
Level at bedtime = Σ dose × 0.5^(hours from that drink to bedtime ÷ half-life)
The page treats each drink as absorbed at once. In reality blood levels peak 30 to 60 minutes after drinking, which barely moves the bedtime figure.

The latest another coffee fits

The page also works backward. Taking what your earlier drinks will leave at bedtime, it finds the latest time one more 95 mg cup could go down and still leave you under 50 mg when you go to bed. If your day already leaves more than that, there is no such time.
Be careful how much weight you put on 50 mg. It is a rule of thumb, not a threshold anyone established in a trial. In the best-known experiment (Drake and colleagues, 2013), 400 mg taken six hours before bed still cost people more than an hour of sleep, and they did not notice.

Half-life by person

Caffeine in the presets

Daily limits: the FDA calls 400 mg a day generally safe for healthy adults. In pregnancy the advice is 200 mg (ACOG and EFSA), and the American Academy of Pediatrics discourages caffeine for children under 12 and suggests no more than 100 mg a day for ages 12 to 18.

The page assumes all the drinks fall on the same day, counted from midnight, and that bedtime comes after the last of them. A bedtime earlier on the clock than your last drink is read as the next morning, so 1:00 after a 21:00 coffee works.