Caffeine Half-Life

Methodology

remaining = dose × 0.5^(hours ÷ half-life), and the share left is remaining ÷ dose. The time until a fraction f is left is half-life × log(f) ÷ log(0.5): 3.32 half-lives for 10% and 6.64 for 1%. Results print to one decimal, and less than 1 mg prints as “< 1”. The page accepts 1 to 1,000 mg, 0 to 48 hours and a custom half-life of 1 to 24 hours. With a dose time and a check time, the hours are the gap between them on a 24-hour clock, and a check time earlier than the dose time is the next day: 21:00 to 07:00 is 10 hours.

Assumptions

• The whole dose counts from time 0. In the study FSANZ cites, caffeine peaked in the blood about 30 minutes after an oral dose, so the first hour reads high.
• One dose only. A second drink is not added; run the page again for it.
• Elimination is first order. FSANZ describes it that way for intakes up to 10 mg per kg of body weight.
• The smoker, pregnancy, oral contraceptive and liver impairment presets are illustrative values, not measurements of anyone.

Frequently asked questions

How long does caffeine stay in your system?

With a 5-hour half-life, 10% of a dose is left after 16.6 hours and 1% after 33.2 hours. With 3 hours, the low end of the adult range FSANZ gives, those times are 10.0 and 19.9 hours; with 7 hours, the high end, 23.3 and 46.5 hours. The FDA notes wide variation in how fast people eliminate caffeine.

If I drink coffee at 3 pm, how much caffeine is left at bedtime?

Enter the dose and either the hours until bed or the two clock times. 200 mg at 3 pm leaves 66.0 mg at 11 pm with the 5-hour default, about a third of the dose, and the brewed-coffee button (180 mg) with 15:00 and 23:00 leaves 59.4 mg. The FDA lists 113 to 247 mg for 12 fl oz of brewed coffee, so check what your cup holds. The page sets no bedtime limit, because sensitivity varies from person to person; use the figure to compare an earlier or smaller cup rather than as a limit.

How much caffeine a day is too much?

The FDA has cited 400 mg a day for most adults as an amount not generally associated with negative effects, and advises talking to a health care provider if you are pregnant, trying to become pregnant or breastfeeding. ACOG Committee Opinion 462 (2010) found that less than 200 mg a day in pregnancy does not appear to be a major factor in miscarriage or preterm birth.

Does pregnancy slow down caffeine?

Yes, as pregnancy goes on. FSANZ reports a half-life in the first trimester similar to that of women who are not pregnant, about 10 hours by week 17 and up to 18 hours by the end of pregnancy. The page’s third-trimester preset uses 18 hours: 95 mg is still 88.0 mg after 2 hours. For another stage, enter the figure under Custom.

How much caffeine is in a cup of coffee or tea?

The FDA gives 113 to 247 mg for 12 fl oz of regular brewed coffee, 71 mg for black tea, 37 mg for green tea, 23 to 83 mg for caffeinated soft drinks and 41 to 246 mg for energy drinks. The drink buttons fill in those figures, or the middle of a range: 180 mg for coffee, 53 mg for a soft drink and 144 mg for an energy drink. Your drink can hold more or less, so check its label.

Do birth control pills change caffeine half-life?

They can lengthen it. FSANZ cites a study in which women taking oral contraceptives had a caffeine half-life of 10.7 hours against 6.2 hours without, and another that found 7.88 hours against 5.37. The page’s oral contraceptive preset is 10 hours and is illustrative.