How to use the caffeine calculator
- Choose a drink and the time you had it, then press Add drink. Repeat for each coffee, tea, cola or energy drink today.
- Set your bedtime and the caffeine half-life that fits you.
- See how much caffeine is still active at bedtime, your total for the day and a chart of the level over 24 hours.
Caffeine half-life explained
Your liver breaks caffeine down at a roughly steady proportional rate. The half-life, the time it takes for half of it to be cleared, is about 5 hours for a typical healthy adult, but ranges from about 3 hours in smokers to 10 hours or more for people taking oral contraceptives, and up to 15 hours late in pregnancy. Some people also carry slower versions of the CYP1A2 enzyme that metabolises it. With a 5-hour half-life, a 200 mg coffee at 8 am still leaves 50 mg in your system at 6 pm and about 25 mg at 11 pm.
The calculator adds up every drink using exponential decay: remaining = dose × 0.5(hours since drinking ÷ half-life). It treats caffeine as absorbed immediately, although levels actually peak 30–60 minutes after drinking. Sleep studies have found that 400 mg taken even six hours before bed noticeably shortens sleep, so many sleep experts suggest a caffeine cut-off around noon to 2 pm.
Your drinks are saved only in this browser so the list is still there when you come back; nothing is uploaded.
How long does caffeine stay in your system?
It takes about five half-lives to clear almost completely: roughly 25 hours at a typical 5-hour half-life, though effects fade much sooner.
How much caffeine is too much?
For healthy adults, up to 400 mg a day is considered safe by the FDA and EFSA; 200 mg during pregnancy.
Does decaf have caffeine?
A little: usually 2–15 mg per cup.