How the caffeine half-life calculation works
Caffeine leaves your body exponentially: every half-life — about 5 hours for the average adult — half of what remains is metabolized. The formula is C(t) = dose × 0.5^(t ÷ half-life). So a 200 mg pre-workout at noon is still 100 mg at 5 PM, 50 mg at 10 PM, and 25 mg at 3 AM. Multiple drinks stack: this calculator sums the remaining caffeine from every drink you add.
Why 30 mg is the "sleep-safe" line
There's no magic number, but sleep research generally finds that under roughly 30–50 mg — about a third of a cup of coffee — most people can fall asleep normally. Above ~100 mg at bedtime, expect longer time to fall asleep, less deep sleep, and more wake-ups, even if you do drift off. Pair this tool with our Sleep Cycle Calculator to plan the whole night.
What changes your personal half-life?
- Faster clearance (3–4 hrs): regular smokers, some genetic fast metabolizers.
- Average (5 hrs): most healthy adults.
- Slower (6–10+ hrs): pregnancy, oral contraceptives, some medications, liver conditions, and slow-metabolizer genetics.
If coffee at noon wrecks your sleep, you're probably on the slow end — set the half-life higher in Options and watch how the curve changes.
How much caffeine is safe per day?
The FDA cites 400 mg per day as a level generally not associated with negative effects for healthy adults — roughly four 8-oz cups of brewed coffee. Pregnant people are typically advised to stay under 200 mg. This calculator is for general information only; talk to your doctor about your own situation.