Caffeine Half-Life Estimator
Quick answer: A 200 mg coffee with a 5.5-hour half-life leaves about 44.1 mg (22.0%) still in your body 12 hours later, and reaches under 25 mg about 16.5 hours after the dose. The estimator repeats this exponential-decay math with your dose, elapsed time, and an editable half-life estimate.
How much caffeine is still in your body hours after that coffee? This caffeine half-life estimator applies the exponential-decay model to your dose and elapsed time, showing estimated remaining milligrams, the percent of the dose left, and when you drop below 25 mg.
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How to use this calculator
Enter the caffeine in milligrams from your drinks (coffee, tea, energy drinks, soda), the hours since you had them, and your assumed half-life. The default 5.5 hours is an estimate for healthy adults — change it if you have a personal figure. Results update instantly as you type.
How this is calculated
This tool applies the exponential-decay (half-life) model. Formula: remaining = dose x 0.5^(t / half-life), where t is hours elapsed. Percent remaining = remaining / dose x 100. Time until under 25 mg (from dose) = half-life x log2(dose / 25); subtract elapsed time to get the remaining wait from now.
- Inputs: caffeine consumed in milligrams; hours elapsed since consuming; caffeine half-life in hours — an estimate you can edit.
- Assumptions: a single dose taken at time zero; clearance follows a constant half-life; the default 5.5-hour half-life is an estimate, not your personal clearance rate.
- Edge cases: if the dose is at or below 25 mg, no wait time is shown; entering a different half-life rescales every output.
- Estimates: all outputs are labeled estimates. The model does not replace medical measurement or advice.
Reference data
| Figure | Value | Notes |
|---|---|---|
| Caffeine half-life, healthy adults | 5-6 hours (calculator default: 5.5) | Commonly cited range; varies by individual (genetics, pregnancy, medication) |
| Reference dose in worked example | 200 mg | User-entered, computed live |
Source: Sleep Foundation (2026) — half-life figures are labeled estimates, not measurements of any individual. The calculator itself is a math model; every result is computed live from your inputs.
Method & sources
The only external figure on this page is the 5-6 hour half-life range for healthy adults, sourced from the Sleep Foundation (2026) and labeled an estimate. Everything else is computed live from your inputs with the exponential-decay formula above.
Worked example
200 mg consumed, 12 hours elapsed, 5.5-hour half-life. Remaining: 200 x 0.5^(12 / 5.5) = 200 x 0.5^2.1818 = 44.1 mg, which is 22.0% of the dose. Time until under 25 mg from dose: 5.5 x log2(200 / 25) = 5.5 x 3 = 16.5 h; with 12 h elapsed, about 4.5 h from now.
Frequently asked questions
What does caffeine half-life mean?
The half-life is the time it takes your body to clear half of the caffeine in it. With a 200 mg dose and a 5.5-hour half-life, about 100 mg remains after 5.5 hours and about 50 mg after 11 hours — each half-life removes half of what is still there, not half of the original dose.
Why is the half-life a range instead of one exact number?
Caffeine clearance varies by individual — genetics, pregnancy, some medications, and liver function all change it. Published estimates commonly cite 5-6 hours for healthy adults, so this calculator uses 5.5 hours as an editable default estimate, not a measurement of your body.
Is this calculator medical advice?
No. It is an informational estimate built on a simple exponential-decay model. It does not diagnose or treat anything. Talk to a clinician if you are managing sleep, anxiety, pregnancy, or caffeine interactions with medication.
How accurate is the remaining-caffeine estimate?
It is a model, not a measurement. It assumes a single dose taken at time zero and a constant half-life. Real clearance can run faster or slower, and several servings through the day add up, so treat the number as a rough estimate.