How to use the Caffeine Calculator
- Enter the drink, number of servings and the time. You can add up to four entries; choose Custom if a drink is not listed.
- Set your bedtime, the half-life and a threshold amount. Start with 5 hours and adjust; it ranges from about 3 to 7 hours between people.
- See your total for today, the caffeine left at bedtime, the time it drops below the threshold, and the curve on the chart.
How it works — Caffeine Calculator
Caffeine is mostly absorbed within about an hour and then the body clears half of what remains in a fixed amount of time, called the half-life. This calculator assumes each dose is fully absorbed at the time you drink it and adds up each dose multiplied by 0.5 raised to the power of elapsed time divided by half-life. For example, 150 mg at 9:00 with a 5-hour half-life leaves 75 mg at 14:00 and 37.5 mg at 19:00. With several drinks, each remaining amount is added together. Half-life varies a lot with metabolism, medication, smoking and pregnancy.
How much is reasonable
Health authorities such as the Korean Ministry of Food and Drug Safety advise healthy adults to stay under 400 mg of caffeine a day, with lower limits for pregnant people, children and teenagers. How much affects sleep varies widely, so the threshold in this tool is a value you choose, not a medical standard. If you struggle to fall asleep, look at the amount left at bedtime and try moving your last drink earlier.
Tips
- A simple first step is to cut caffeine about six hours before bed.
- Coffee varies by cup size and number of shots, so use the custom field when you know the amount.
- Energy drinks also contain a lot of sugar, so keep that in mind.
- Try half-lives of 3 and 7 hours to see the range that may fit you.
Notes & limitations
This calculator provides general health information and is not a diagnosis or treatment. If you are pregnant, have a heart condition or insomnia, or take medication, talk with a doctor or pharmacist. The caffeine amounts per drink are rough averages.
FAQ
What half-life should I use?
Healthy adults are typically around 5 hours, but it ranges from about 3 to 7 hours. Start with 5 and adjust.
Does decaf contain caffeine?
Yes, a small amount, usually a few to about ten mg per cup, so it is listed separately.
Is 400 mg a day safe?
It is a general upper guideline for healthy adults and individual differences are large. Cut back if you feel unwell.