What this calculator does
Caffeine clears by first-order elimination, halving at a steady interval. With an average adult half-life of about five hours, a 200 mg coffee still leaves 100 mg in circulation five hours later and 50 mg after ten.
That arithmetic explains afternoon coffee. A cup at 3pm leaves around a quarter of its caffeine at 1am, which is enough to affect sleep quality in many people even when falling asleep is not a problem.
The formula
The dose is multiplied by one half raised to the power of elapsed time divided by half-life. It is the same exponential decay that governs drug elimination generally.
| Term | Meaning |
|---|---|
| Half-life | The time for the amount in the body to fall by half, averaging about 5 hours for caffeine. |
| First-order elimination | Clearance proportional to the amount present, producing exponential decay. |
| Individual variation | Caffeine half-life ranges roughly from 1.5 to 9.5 hours between people. |
The inputs explained
| Field | What to enter |
|---|---|
| Caffeine consumed (mg) | Caffeine consumed in milligrams. A typical brewed coffee is 80 to 120 mg. |
| Assumed half-life (hours) | Assumed half-life in hours. Five is the usual average, but individuals vary widely. |
| Time since consumption (hours) | Hours since consumption. |
When to use it
Working out a coffee cut-off time
Counting back from bedtime shows how much would still be present.
Understanding why sleep is affected
A quarter of an afternoon dose is commonly still present at bedtime.
Planning caffeine around performance
The decay curve shows when levels peak and fade relative to an event.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much is left as the hours pass?
The same dose at four points afterwards.
| Time since consumption | Caffeine remaining | Share of dose remaining |
|---|---|---|
| 0 hours | 200 mg | 100.0% |
| 5 hours | 100 mg | 50.0% |
| 10 hours | 50 mg | 25.0% |
| 15 hours | 25 mg | 12.5% |
Questions
Why does caffeine half-life vary so much?
Mainly because of the liver enzyme CYP1A2, whose activity differs genetically between people. Pregnancy roughly doubles the half-life, some oral contraceptives extend it substantially, and smoking shortens it.
Does caffeine still affect sleep if I fall asleep fine?
It can. Research has found caffeine reduces deep sleep and total sleep time even in people who report no difficulty falling asleep, so the absence of obvious wakefulness is not evidence that it has cleared.
Does tolerance change how fast it clears?
No. Regular use produces tolerance to the subjective effects, but elimination speed is unchanged. The same amount is present in the blood; it simply feels different.
How much is too much?
Up to about 400 mg a day is generally considered acceptable for most healthy adults, with lower limits in pregnancy. Sensitivity varies considerably, and anyone with a heart condition or anxiety should ask their doctor.
For drug elimination generally, see the drug half-life calculator. For alcohol clearance, see the blood alcohol content calculator.