☕ Caffeine Half-Life Calculator
Caffeine's half-life varies significantly between individuals — around 5 hours is a common average.
Enter the amount of caffeine consumed (mg), time elapsed, and caffeine's half-life to calculate how much remains in your body, based on first-order elimination kinetics (the amount halves at a regular interval). Caffeine's half-life varies significantly between individuals, but is commonly cited as around 5 hours on average.
How to use
- Enter the amount of caffeine consumed (mg).
- Enter the time elapsed since consumption.
- Enter caffeine's half-life (hours) — around 5 hours is a common average if unknown.
How the calculation works
This tool estimates how much caffeine is still in your body using its half-life. Remaining (mg) = amount taken × 0.5^(hours elapsed ÷ half-life) Time to fall below 10% = half-life × log₂10 ≈ half-life × 3.32 Caffeine is broken down mainly in the liver and declines roughly by first-order kinetics: the amount halves every fixed interval. The average half-life in healthy adults is about 5 hours, but it varies widely, from about 2 to 10 hours. Smoking shortens it; pregnancy and some medications lengthen it. Caffeine still in your system at bedtime can make it harder to fall asleep and lighten your sleep.
Worked example
One cup of coffee (about 95 mg), 8 hours later, 5-hour half-life Remaining: 95 × 0.5^(8 ÷ 5) ≈ 31 mg Time to fall below 10% (9.5 mg): 5 × 3.32 ≈ 16.6 hours Coffee at 3 pm still leaves nearly 30% (about 27 mg) in your body at midnight (9 hours later).
Things to be aware of
- Up to 400 mg of caffeine a day (about four cups of coffee) is considered safe for healthy adults (European Food Safety Authority, Health Canada and others).
- During pregnancy, keeping to 200–300 mg a day or less is recommended.
- Caffeine is also found in green and black tea, energy drinks, chocolate and some cold medicines.
FAQ
What is a half-life?
The time it takes for the amount of a substance in the body to reduce by half. Caffeine follows first-order elimination kinetics, so the amount in your body roughly halves at each half-life interval.
How long is caffeine's half-life?
Around 5 hours on average for a healthy adult, but it varies significantly based on genetics, smoking, pregnancy, and certain medications. This tool lets you set any half-life to try different scenarios.
Is this result medical advice?
No — this is a simplified calculation based on first-order kinetics, intended for educational reference. Consult a medical professional for specific decisions about caffeine intake.