How caffeine clears your body
Caffeine leaves the body through exponential decay, which means a fixed fraction is removed over each equal stretch of time rather than a fixed amount. The half-life is the time it takes for the amount in your system to halve.
With a typical half-life of about five hours, a 200 mg dose falls to roughly 100 mg after five hours, 50 mg after ten, and 25 mg after fifteen. The calculator applies this curve to the dose and time you enter and projects when the level drops below 50 mg.
Why your half-life may differ
Five hours is only an average. How quickly you clear caffeine depends largely on liver enzymes and other factors, so the same coffee can keep one person wired and barely touch another.
- Pregnancy can roughly double caffeine half-life.
- Some medications and oral contraceptives slow clearance considerably.
- Smoking speeds clearance, shortening the half-life.
- Genetics make some people fast metabolisers and others slow ones.
Using this for better sleep
Because caffeine lingers for many hours, an afternoon coffee can still be active at bedtime. The chart shows the falling curve so you can see how much remains when you plan to sleep.
A common guideline is to stop caffeine at least six to eight hours before bed, and longer if you are sensitive or a slow metaboliser.
A note on health
This tool is an educational estimate, not medical advice. Caffeine affects heart rate, blood pressure and anxiety in some people. If you are pregnant, take medication, or have a heart condition, ask a healthcare professional about a safe daily limit.
Formula
remaining = dose · 0.5^(hours / half-life)Frequently asked questions
- How long does caffeine stay in your system?
- With a 5-hour half-life, a 200 mg dose falls to about 100 mg after 5 hours and 50 mg after 10 hours. It takes roughly a full day to clear almost completely.