Caffeine Calculator: How Long Will Caffeine Keep You Awake?

The model behind the number#

Caffeine is almost completely absorbed from the gut, peaks in the blood 30 to 60 minutes after you drink it, and is then cleared at a constant proportional rate. That last property is what makes the arithmetic simple: every half-life, whatever is left gets halved again.

The calculator uses the standard one-compartment decay:

Remaining = dose × 0.5 ^ (hours ÷ half-life)

At a 5-hour half-life, 200 mg becomes 100 mg after five hours, 50 mg after ten, and 25 mg after fifteen. There is no point at which caffeine abruptly "wears off" — the phrase describes the moment you stop noticing it, which arrives long before the molecule is gone.

Why 30 mg is the cut-off#

Most caffeine calculators pick a threshold that sounds sensible and never justify it. This one derives it.

The 2023 meta-analysis by Gardiner and colleagues pooled controlled trials to answer a practical question: how many hours before bed does a given dose need to be taken so that sleep is not measurably affected? Two of their answers are 8.8 hours for 100 mg and 13.2 hours for 217 mg.

Put both through the decay equation with a 5-hour half-life and you get the amount still circulating at those cut-offs:

Dose	Published cut-off	Still in the body at that point
100 mg	8.8 h before bed	29.5 mg
217 mg	13.2 h before bed	34.8 mg

Two very different doses and two very different cut-off times converge on roughly 30 mg. That is the threshold the calculator uses, which is why its answers track the published guidance instead of competing with it.

What changes your half-life#

The five-hour figure is a population average that hides a very wide spread. The reported range in healthy adults is 2 to 10 hours, and the main driver is the CYP1A2 gene, which controls the liver enzyme responsible for about 95% of caffeine clearance.

• Smoking induces that same enzyme and cuts the half-life to roughly 3 hours. Quitting reverses it within about a week — which is one reason newly ex-smokers often feel jittery on their usual coffee.
• Combined hormonal contraceptives inhibit it, roughly doubling the half-life to around 7–10 hours.
• Pregnancy slows clearance progressively; by the third trimester half-lives above 15 hours have been recorded.
• Liver disease and some medications — including certain antibiotics and antidepressants — extend it further.

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The 200 mg question specifically#

"How long will 200 mg keep me awake" is the most common version of this question, and it deserves a precise answer, because the honest one is not the one people expect.

At a 5-hour half-life, 200 mg falls below 30 mg after 13.7 hours. Drink it at 2 pm and it is still above the threshold at midnight. Drink it at 9 am and you are clear by around 10:45 pm.

But "keep you awake" is the wrong frame. You will very likely fall asleep. The 2013 study by Drake and colleagues gave people 400 mg at bedtime, three hours before bed and six hours before bed, and measured sleep objectively. All three conditions disrupted sleep — including the dose taken six hours ahead, which participants did not perceive as having any effect at all. What caffeine takes from you at that distance is not consciousness. It is deep sleep.

What the calculator does not know#

It does not know your genotype, so the half-life you pick is a category rather than a measurement. It does not model tolerance, adenosine receptor upregulation, or the interaction with alcohol. It assumes a single dose rather than a day of sipping — for several drinks, add up the doses and use the time of the last one, which slightly understates the total.

And it says nothing about whether caffeine is a problem for you. If you sleep seven to eight hours, wake unprompted and function without a morning dose to feel normal, the number in this calculator is a curiosity. If you need coffee to start and cannot switch off at night, it is the loop worth breaking — and moving the cut-off earlier is the cheapest intervention available.