MATH · FREE
Caffeine Calculator
List what you drank and when. The calculator adds up the caffeine still in your body right now and at bedtime, using a half-life you can change.
Math only. Not medical advice. Caffeine clearance varies between people and changes with pregnancy, medicines and other factors. Talk to a clinician about your own limits.
CAFFEINE LEFT AT BEDTIME
Left right now
Total drunk today
Share left at bedtime
Math only. Not medical advice. Real caffeine clearance varies between people and changes with pregnancy, medicines and other factors. This estimate assumes simple halving every half-life.
How the estimate works
For each drink the calculator uses N = mg × (1/2)hours since the drink ÷ half-life, then adds the drinks together. A drink you have not had yet (a time later than "now") counts at bedtime but not in the "right now" number.
Worked example: two coffees
Two 180 mg coffees, one at 08:00 and one at 13:00, with a 5 hour half-life and a 23:00 bedtime. The first has had 15 hours, so 180 × (1/2)3 = 22.5 mg is left. The second has had 10 hours, so 180 × (1/2)2 = 45 mg is left. Together that is 67.5 mg of the 360 mg, or 18.75%. At 15:00 the first has had 7 hours (about 68.2 mg) and the second 2 hours (about 136.4 mg), so about 204.6 mg is in the body.
Why the half-life changes the answer so much
The same two coffees leave about 23.5 mg at bedtime with a 3 hour half-life and about 124.8 mg with an 8 hour half-life. Because the amount halves repeatedly, a few hours of difference in the half-life makes a large difference late in the evening. Use the number that fits what you know about yourself, and treat the result as a rough picture, not a measurement.
Sources
- FDA, Spilling the Beans: How Much Caffeine Is Too Much? (accessed October 2026). Caffeine per 12 oz for brewed coffee, tea, soft drinks and energy drinks, and the 400 mg a day figure. The FDA page has no espresso figure, so espresso is not a preset.
- Struniewicz et al., Pregnancy and Caffeine Metabolism, Nutrients 2025 (PMC12526311) (accessed October 2026). Average caffeine half-life of approximately 4 to 5 h in healthy adults; prolonged in pregnancy, up to approximately 15 h in the late gestational trimester.
- Song et al., Caffeine: A Multifunctional Efficacious Molecule, Int. J. Mol. Sci. 2024 (PMC11593559) (accessed October 2026). Gives a caffeine half-life of 3 to 5 h.
Frequently asked questions
How does the caffeine calculator estimate what is left?
Each drink is treated as a separate amount of caffeine that halves every half-life, starting when you drank it. The amounts left from all drinks are added up. With a 5 hour half-life, a 180 mg coffee at 08:00 leaves 90 mg at 13:00 and 45 mg at 18:00.
Which half-life should I use?
A 2025 review in Nutrients states that in healthy adults the average half-life of caffeine is approximately 4 to 5 hours, and a 2024 review gives 3 to 5 hours. The calculator starts at 5 hours. Try 3 and 5 to see the spread. In late pregnancy the half-life can be up to about 15 hours, so set it higher if that applies.
How much caffeine is in my drink?
The FDA lists 113 to 247 mg per 12 oz of regular brewed coffee, 23 to 83 mg per 12 oz of caffeinated soft drinks, 41 to 246 mg per 12 oz of energy drinks, 71 mg for black tea and 37 mg for green tea per 12 oz. The presets use the middle of a range where one is given. Check your label and edit the mg.
Is there a safe cutoff time for caffeine?
This page does not set one. It shows how much of what you drank is estimated to remain at the bedtime you choose, so you can compare days. How sensitive you are to that amount differs from person to person.
How much caffeine per day is too much?
The FDA cites 400 mg a day as an amount not generally associated with negative effects for most healthy adults. The total drunk today appears in the results so you can compare. Pregnancy, medicines and health conditions change what is right for you.
Math only. Not medical advice. Caffeine clearance varies between people and changes with pregnancy, medicines and other factors. Talk to a clinician about your own limits.
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