How it is calculated
Caffeine left = Σ Dose × 0.5(Hours since drink ÷ Half-life)
Example: Two cups of coffee (95 mg each) at 8:00 AM and 2:00 PM, with a 5-hour half-life, leave about 42 mg in your body at 10:30 PM — more than a fifth of what you drank. The level fell below 50 mg at about 9:15 PM.
Sleeping better
- Move your last caffeinated drink earlier in the day.
- Watch for hidden caffeine in tea, chocolate, pre-workout and some pain relievers.
- If you are sensitive, try the “slow” half-life setting.
Brewing at home? Get the ratio right with the coffee ratio calculator.
Frequently asked questions
What is the half-life of caffeine?
About 5 hours on average in healthy adults, but it ranges from roughly 2 to 10 hours. Smoking shortens it; pregnancy and oral contraceptives lengthen it.
How much caffeine is in coffee?
An 8-oz (240 ml) cup of brewed coffee has about 95 mg, a shot of espresso about 63 mg, black tea about 47 mg and a 12-oz cola about 35 mg. Energy drinks range from 80 to 300 mg.
How much caffeine is safe per day?
The U.S. FDA says up to 400 mg a day is generally not associated with negative effects in healthy adults. During pregnancy the usual advice is 200 mg or less.
When should I stop drinking coffee before bed?
A study in the Journal of Clinical Sleep Medicine found 400 mg taken even 6 hours before bed reduced sleep by over an hour. Many experts suggest a cutoff 8 hours before bedtime.