How the decay estimate works
Caffeine clears the body through first-order elimination: a fixed fraction is removed per unit time, so the amount remaining halves every half-life. The average half-life is about 5 hours, but the calculator lets you adjust it for your own metabolism.
- C₀ — initial caffeine dose (mg)
- t — time elapsed (hours)
- halfLife — hours for the amount to halve (default 5)
More detail
Why half-life varies so much
A half-life of 5 hours is a population average — individual values range from about 1.5 hours (fast metabolizers) to 9.5 hours (slow metabolizers, often due to genetics, pregnancy, or liver enzyme differences). Adjust the half-life input to match your own experience if you know it.
Why 10% matters for sleep
The "time to drop below 10%" output marks when caffeine's stimulant effect is largely gone. At a 5-hour half-life this is about 16.6 hours after intake — which is why an afternoon coffee can still measurably affect sleep that night.
Practical tip. If caffeine reliably disrupts your sleep, try the calculator with a longer half-life (7–9h) — some people genuinely clear caffeine more slowly, and cutting off intake earlier in the day matters more for them.
Frequently asked questions
How long does caffeine take to leave your system?
With the average 5-hour half-life, caffeine drops below 10% of the original dose after about 16.6 hours (halfLife × log₂10). The "Time to drop below 10%" output above computes this for your chosen half-life.
How much caffeine is left after 8 hours?
It depends on the dose and half-life you enter — with a 200mg dose and the default 5-hour half-life, about 200 × 0.5^(8/5) ≈ 66mg remains. Enter your own numbers above to see the exact figure.
Why does the calculator let me change the half-life?
Caffeine half-life isn't fixed — it ranges from roughly 1.5 to 9.5 hours between individuals depending on genetics, liver function, pregnancy, and other factors, so the default of 5 hours is only a population average you can override.
Does this account for how caffeine is absorbed, not just eliminated?
No — this models only the elimination phase (first-order decay after peak absorption), which is standard for half-life calculations. It doesn't model the initial absorption curve, so results are most accurate starting from roughly 30–60 minutes after intake.