Caffeine Cutoff Calculator

What It Does

This page reads one relation — level(t) = dose × 0.5^(t ÷ halfLife) — for the two questions a dose of caffeine raises in the evening. It gives the amount still present at bedtime and the share of the dose that represents, and it gives the cutoff: the latest time of day the dose can be taken and still leave your chosen level at bedtime. Beside those it prints the clock hour at which the level falls to that level, which is the same decay counted forward from the dose instead of backwards from bedtime.

Real-World Usage

The page is for the evening question: whether the coffee you had after lunch is still working when you go to bed, and when you would have had to stop. It is also a way to see why the same dose behaves differently between people — moving the half-life from 5 hours to 3 turns 32.99 mg at bedtime into 15.75 mg — and a way to compare drinks, once the caffeine in each is known in milligrams. Shift workers and anyone whose bedtime moves can read it against the bedtime they actually have rather than a fixed hour.

Local Context

The elimination of caffeine is a property of the body and not of the country, so nothing here changes with your location and one page serves all nine countries with the same arithmetic. Two conventions are worth knowing. The decimal separator in the input fields is the point, as in 1.5 and 0.25, whatever separator your language uses in ordinary prose; every figure the page prints is formatted in your language's own convention, so the same level reads 32,99 in German and 32.99 in English, and the clock follows your locale's own 12- or 24-hour reading. And what is in a drink is not fixed by law: a coffee or a cola varies by brand and by how it is made, so the milligram figure you enter is the one figure the page cannot check for you.

How long does caffeine stay in your body?

It depends on the dose and on your half-life, and it never quite reaches zero. With a 5-hour half-life, 200 mg is down to 100 mg after 5 hours, 50 mg after 10, 25 mg after 15 and about 6 mg after 25 — a little under 3 % of what you took. A longer half-life moves every one of those hours later, and a shorter one moves them earlier.

What should I put for the half-life?

Five hours is the quoted mean for healthy adults, and the range usually given is 1.5 to 9.5 hours, so it is worth trying both ends of that range as well as the mean. Smoking roughly halves it; pregnancy, oral contraceptives and some medicines lengthen it. The field is yours to set, and the figures here are only as good as the half-life you enter.

Why does the page ask for a level rather than a time before bed?

Because a fixed hour before bed is not the same rule for everyone. What matters is how much is left when you go to bed, and that depends on the dose: 400 mg taken 6 hours before bed leaves more than 96 mg, while 50 mg taken at the same hour leaves about 12 mg. Setting the level you want makes the page answer for your own dose.

Does the page work for tea, cola and energy drinks?

Yes, if you know the amount in milligrams. A mug of filter coffee is often around 100 to 140 mg, a shot of espresso around 45 mg, a cup of tea around 40 to 50 mg and a can of cola around 40 mg, but the figures vary by brand and by how the drink is made, so a label or a published table is a better input than a guess.