Caffeine Cutoff Time Calculator

How it is calculated

Caffeine is modelled as exponential decay, halving every half-life. The tool works out how many half-lives it takes for the dose to fall to the amount you accept at bedtime: hours = half-life × log2(dose ÷ threshold). The cutoff is bedtime minus those hours, rounded to the minute and shown as a 24-hour clock time. Bedtime is typed as HH:MM; dose and threshold are in mg and the half-life is in hours (above 0, at most 12). The dose must be above 0 and at most 1000 mg, and the threshold must be above 0 and below the dose, otherwise there is no result. If the wait is 24 hours or more, the clock wraps and the result adds how many days back it is, for example (−4 d). The threshold is your own choice, not a medical limit, and only one dose is modelled.

Common mistakes

• Entering the volume of the drink instead of its caffeine in mg. A 250 ml cup is not 250 mg; the field wants the milligrams of caffeine in it.
• Setting the threshold equal to or above the dose. The threshold is what is left at bedtime and must be lower than what you drank, or there is no result.
• Entering a half-life above 12 hours, or a dose above 1000 mg. These are rejected and show no result rather than a cutoff.
• Typing bedtime in a 12-hour style such as 11pm. The format is 24-hour HH:MM, for example 23:00, and 11pm shows no result.
• Choosing a tiny threshold and being surprised by the answer. A threshold of 0.0001 mg gives 99.3 hours, which the tool shows as a time of day plus the number of days back, here 19:43 (−4 d).