Caffeine Calculator
Estimates how much caffeine is still in your body when you go to bed.
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The formula
remaining = dose × 0.5^(hours ÷ half-life)
Half-life is the time your body takes to clear half of the caffeine. It is about 5 hours on average, and commonly runs from about 2 to 10 hours.
What to watch out for
Caffeine leaves the body gradually, so an afternoon coffee can still be working at bedtime. Half-life varies widely: smoking shortens it, while pregnancy, some medicines and the contraceptive pill lengthen it, and genes play a part too. Try a higher or lower half-life to see the range.
Healthy adults are usually advised to stay under 400 mg a day, and under 200 mg in pregnancy. If you sleep badly even with little caffeine, it is worth raising with a doctor.
Frequently Asked Questions
How much caffeine is left after 5 hours?
At a 5-hour half-life, 100 mg of a 200 mg dose. After 10 hours it is 50 mg.
When should I stop drinking coffee?
It depends on your sensitivity. A 200 mg coffee at 15:00 with a 5-hour half-life leaves 66 mg at 23:00, and 38 mg if you have it at 11:00 instead.
What if my half-life is longer?
The same 200 mg at 15:00 leaves 100 mg at 23:00 with an 8-hour half-life, against 31 mg with a 3-hour one.