Caffeine Half-Life Calculator
Estimate caffeine remaining after a given time and when it falls below a target.
What it does
Caffeine is a stimulant found in coffee, tea, soda, chocolate, and supplements. Its effect on sleep and alertness depends on how much is in the body, not only on how long ago you drank it. A half-life calculator turns that idea into a useful estimate.
The body clears caffeine roughly by first-order elimination. In simple terms, a fixed fraction leaves per hour, so the amount remaining follows an exponential curve. One half-life removes half of what is present, the next half-life removes half of the rest, and so on.
With the defaults of 200 mg, 5 hours since the dose, and a 5 hour half-life, 100 mg remains. That is 50.0 percent of the original dose. The same dose falls to 50 mg at 10 hours and 25 mg at 15 hours. A second cup adds a new curve on top of the first.
Caffeine half-life is not the same for everyone. Reviews often report about 3 to 7 hours in healthy adults, with an average near 5 hours. Smoking can speed clearance, while pregnancy, liver disease, and some medicines can slow it. Genetics also matter.
The FDA notes that 400 mg of caffeine a day is an amount not generally associated with negative effects in healthy adults, but sensitivity and medical conditions change that guidance. This page is a planning estimate, not medical advice, and it does not replace a clinician who knows your health history.
How to use
- Enter the amount of caffeine in milligrams.
- Enter the hours since you finished the dose.
- Enter a half-life, or leave the default 5 hours.
- Press Calculate to update the estimate.
- Read the remaining milligrams and the share of the dose.
- Check the times to 100 mg and 50 mg if you care about sleep.
Formula
- C_0
- Starting caffeine dose in milligrams.
- t
- Hours since the dose.
- t_half
- Caffeine half-life in hours.
- target
- The chosen remaining amount, 100 mg or 50 mg.
The model assumes one dose at time zero and a constant half-life. It ignores absorption, so the peak may arrive 30 to 60 minutes after a drink. For 200 mg and a 5 hour half-life, 5 hours gives 200 x 0.5 = 100 mg. The time to 50 mg is 5 x log(200 / 50) / log(2) = 10 hours. Times shown for 100 mg and 50 mg are measured from the dose, not from the current moment.
Worked example
Example: 200 mg with a 5 hour half-life
Defaults: 200 mg dose, 5 hours elapsed, 5 hour half-life.
- Half-lives elapsed: 5 / 5 = 1.
- Remaining: 200 x 0.5^1 = 100 mg.
- Percent: 100 / 200 x 100 = 50.0 %.
- Time to 100 mg: 5 x log(200 / 100) / log(2) = 5 h.
- Time to 50 mg: 5 x log(200 / 50) / log(2) = 10 h.
- Tool shows 100 mg, 50.0 %, 5 h, and 10 h.
Reference table
| Hours after dose | Caffeine left | Share of dose |
|---|---|---|
| 0 | 200 mg | 100.0 % |
| 5 | 100 mg | 50.0 % |
| 10 | 50 mg | 25.0 % |
| 15 | 25 mg | 12.5 % |
| 20 | 12.5 mg | 6.3 % |
| 25 | 6.25 mg | 3.1 % |
Limits
- The model covers one dose and does not add later cups automatically.
- It ignores absorption time, so it does not predict the peak right after a drink.
- Half-life varies widely between people and can exceed 24 hours in some conditions.
- Caffeine from tea, soda, chocolate, and supplements still adds to the total.
Common mistakes
- Using the time since the first sip as if the whole dose arrived at once.
- Assuming a 5 hour half-life fits every person and every day.
- Forgetting caffeine in tea, soda, chocolate, and pre-workout products.
- Adding a second dose without recalculating the first one.
- Treating the estimate as medical advice or as a safe driving limit.
Try it yourself
Quick checks. Answers sit tucked under each question.
100 mg, 10 hours later, 5 hour half-life?
25 mg remains, or 25.0 % of the dose, and the dose reaches 50 mg at 5 h.
400 mg, right after drinking, 6 hour half-life?
400 mg remains, or 100.0 %, with 100 mg reached at 12 h and 50 mg at 18 h.
200 mg, 15 hours later, 5 hour half-life?
25 mg remains, or 12.5 %, and the dose passed 100 mg at 5 h and 50 mg at 10 h.
Questions
How long does caffeine last?
With a 5 hour half-life, about 3.1 % remains after 25 hours. A 3 to 7 hour half-life is common, so the timeline varies.
What is caffeine's half-life?
About 5 hours on average in healthy adults, with a usual range near 3 to 7 hours.
How much caffeine is in coffee?
The FDA lists about 113 to 247 mg per 12 fl oz of brewed coffee. Tea, espresso, and cold brew differ.
Does caffeine half-life change?
Yes. Smoking can speed clearance, while pregnancy, liver problems, and some medicines can slow it.
Can I use this to plan sleep?
It estimates remaining caffeine, but sensitivity varies. If sleep suffers, move the last caffeine earlier and test your response.
Is 400 mg of caffeine a day safe?
The FDA says 400 mg a day is not generally associated with negative effects in healthy adults, but individual limits differ.
Sources
- FDA: Spilling the Beans, how much caffeine is too much
- PubMed Central: caffeine metabolism and half-life review
- StatPearls: caffeine