How it works
Caffeine clears the body on a first-order curve. Every half-life period - typically about five hours - the amount in your bloodstream is cut in half.
If you take 200 mg of caffeine at noon and your personal half-life is 5 hours, you still have 100 mg at 5 PM, 50 mg at 10 PM, and 25 mg at 3 AM. That last quarter-dose is the main reason an innocent afternoon coffee wrecks sleep without feeling like it should.
amount(t) = dose × 0.5(t / half_life)
For multiple doses, the calculator runs that equation independently for each dose and sums the results across the timeline.
Frequently asked
What half-life should I use?
Five hours is the default for healthy adults. Smokers can try 3–4. If you’re pregnant or on hormonal birth control, push toward 7+ hours - clearance slows dramatically.
Why does afternoon coffee ruin sleep?
Because caffeine keeps doing real work at concentrations far below what you can feel. A 2 PM cup leaves roughly a quarter of the dose in your system at bedtime, blocking adenosine receptors and lengthening sleep latency.
What counts as “cleared”?
There’s no universal definition. This calculator considers anything below 10 mg effectively cleared - too low to noticeably affect alertness or sleep.
How accurate is the dose chart?
It’s a reasonable average, but real-world caffeine content swings widely by brand and brew. A Starbucks brewed coffee runs closer to 180 mg per 12 oz; a McDonald’s the same size is about 110. When precision matters, look up the specific drink.