Caffeine calculator

How is it calculated?

The whole calculation is one equation: remaining = taken × 0.5^(hours elapsed ÷ half-life). The half-life is how long your body needs to clear half a dose. In the sample day, two espressos at 13:00 are 126 mg; with bedtime at 23:00 that's 10 hours, or two half-lives, leaving 31.5 mg.

The half-life is offered as a choice rather than a single number: 3, 5 or 7 hours. That is the range reported in healthy adults, and the difference is personal — smoking shortens it, pregnancy and oral contraceptives lengthen it considerably. Presenting the average as the one truth would mislead a slow metaboliser in exactly the wrong direction.

Exponential decay has one useful property: it is memoryless. Because knowing the amount at bedtime is enough to compute the middle of the night and the morning, with no need to add the drinks up again, the "4 hours after bed" and "8 hours after bed" rows fall out of a single multiplication. The past stops mattering.

Caffeine per serving comes from a fixed table: filter coffee 95 mg, espresso 63 mg, black tea 47 mg, energy drink 80 mg. Turkish coffee is the most variable entry — grind, water ratio and time in the pot change the result noticeably; a mid value is used here.

Two safety limits come from EFSA: for healthy adults, intakes up to 400 mg a day and up to 200 mg in one sitting are considered safe; in pregnancy the daily limit is 200 mg. These are safety numbers, not sleep numbers — a day well under 400 mg can still ruin a night.

The "last coffee" hour answers a different question: how late can a cup of filter coffee be drunk and still be under 50 mg by bedtime. Falling from 95 mg to 50 mg takes 4 hours 38 minutes on a five-hour half-life, so for someone going to bed at 23:00 the answer is 18:22. That hour is independent of what you drank today, because so is the question: "if I had one more coffee, how late could it be?"

How to read your result

The sample day shows why the table isn't enough. The total is 268 mg. That sits comfortably under the 400 mg daily limit and raises no warning, yet 64 mg — roughly two-thirds of a cup — is still present at bedtime. Someone looking only at the daily total would call that day fine; but the number that governs caffeine's effect on sleep is not the total, it's those 64 mg.

The morning coffee's share of the evening is smaller than people assume, and the afternoon one's is larger. In the sample only 12 mg is left at bedtime from the 08:00 filter coffee, while 31.5 mg comes from the two 13:00 espressos and 20 mg from a single 17:00 tea, which puts more than half of the evening load on the afternoon drinks. Moving coffee EARLIER is, for most people, easier and more effective than drinking less of it.

Watch what happens when you change the half-life. The same day ends with far more caffeine at bedtime on a slow metabolism (7 hours) and far less on a fast one (3 hours). "Coffee has no effect on me" and "one cup and I'm up all night" can both be true; that number is much of the difference.

The midnight and morning rows show something else: caffeine does not merely delay falling asleep, it is still there in the middle of the night. In the sample there are 37 mg at 03:00. A third of a cup. At three in the morning. For someone who falls asleep fine yet wakes unrested, the explanation may be sitting in that row.

The 50 mg threshold is our comfort line, not a regulatory limit. It is roughly half a cup of filter coffee, and its purpose is to give a comparable reference, not to forbid anything. Your own threshold may be lower; if you are sensitive to caffeine, moving the "last hour" on this page an hour or two earlier is reasonable.

Sleep isn't caffeine's only effect, but it is the most measurable one. Tolerance develops: in regular drinkers the alertness effect fades while the effect on sleep does not fade to the same degree, so the evening coffee drunk under the feeling that "it doesn't affect me any more" can still shorten the night. Losing the feeling is not losing the effect.