What half-life means
A half-life is the time it takes for the amount of caffeine in the body to fall by half. It then takes another half-life to remove half of what remains. So after five hours, half the drink is gone and half is still there.
An Institute of Medicine review reports a mean of about five hours in healthy people, with a range of 1.5 to 9.5 hours. The FDA also notes that people differ in both caffeine sensitivity and how quickly they eliminate it.
Onset and the web calculator use five hours for everyone. That keeps the numbers consistent, but it isn't your personal rate.
One drink, hour by hour
Say you have 100 mg of caffeine at noon and nothing else all day.
- Noon Start 100 mg
- 5 pm 5 hours 50 mg
- 10 pm 10 hours 25 mg
- 3 am 15 hours 12.5 mg
- 8 am 20 hours 6.25 mg
The last two rows are the next morning. These are what the model predicts. They aren't a cutoff for good sleep.
The calculation is amount × 0.5(hours ÷ 5)
. Onset shows a drink as "cleared" once the estimate drops below 5 mg. That's where the app stops counting it. There's still some caffeine left, and 5 mg isn't a safety limit.
In this example, the estimate gets down to 5 mg about 21.6 hours after noon, a little after 9:30 the next morning.
When drinks overlap
Each drink starts wearing off from the time you had it. The calculator works out what's left of each one and adds them up.
Say you have 95 mg at 8 am and another 95 mg at 3 pm. (95 mg is Onset's estimate for an 8 oz coffee.) At 11 pm, about 12 mg is left from the first and about 31 mg from the second, so roughly 43 mg in total.
Try this two-coffee exampleOpens the calculator with this example day selected.
A bigger cup or an earlier time changes the result. If the can or the coffee shop lists the caffeine, use that number instead of a generic entry.
What the estimate can't tell you
The formula doesn't measure anything in your blood. It doesn't know how quickly you absorb a drink, and it can't tell you whether you'll sleep well. It starts the countdown at the time you logged and uses the same rate for every drink.
The FDA advises speaking with a health care provider about caffeine limits if you are pregnant, trying to become pregnant, breastfeeding, or concerned about a medical condition or medication. This guide is general information, not medical advice.
For research about drink timing and sleep, read how many hours before bed to stop caffeine. For an estimate using your drinks and times, use the free caffeine calculator.
Sources and method
- Institute of Medicine, Pharmacology of Caffeine (2001): adult half-life and variation.
- FDA, Spilling the Beans: How Much Caffeine is Too Much?: individual sensitivity and circumstances that call for personal advice.
- Onset calculator method: five-hour exponential decay, summed across drinks. The examples above were calculated with that model.
The numbers in the examples come from the same model as the Onset app.