Caffeine Half-Life Calculator
Estimate how much caffeine is still active at bedtime from the dose, drink time and a half-life you enter, with an hour-by-hour decay table.
In short
Estimate how much caffeine is still active at bedtime from the dose, drink time and a half-life you enter, with an hour-by-hour decay table.
Formula: Remaining = dose × 0.5^(hours elapsed ÷ half-life). This is first-order (exponential) elimination.
What this calculator does
Estimate how much caffeine is still active at bedtime from the dose, drink time and a half-life you enter, with an hour-by-hour decay table.
You enter caffeine consumed, time you had it, bedtime and half-life to assume, and the caffeine half-life calculator shows the answer along with the working, so you can check it or try other numbers.
The page shows the formula, a numeric worked example, and the assumptions that affect this health & fitness calculation.
Inputs and what they mean
- Caffeine consumed
- — Check the label of your drink.
- Time you had it
- — time value.
- Bedtime
- — time value.
- Half-life to assume
- — Published averages for healthy adults are roughly 3–7 hours; enter your own.
How to use it
- Enter each value in the unit shown next to the box (use the unit converter first if your numbers are in other units).
- Check the breakdown to see every intermediate step.
- Read the limitations before relying on the result.
Formula
Assumes the whole dose is absorbed at the time you drink it; absorption actually takes about 30–60 minutes.
Bedtime earlier than the drink time is read as the next day.
Remaining = dose × 0.5^(hours elapsed ÷ half-life). This is first-order (exponential) elimination.
Inputs used: Caffeine consumed, Time you had it, Bedtime, Half-life to assume.
Edge handling: Time you had it must use a valid clock time.; Bedtime must use a valid clock time..
Worked example
120 mg at 3:30 pm, bed at 11:00 pm, 5-hour half-life
- Elapsed = 7.5 h.
- 7.5 ÷ 5 = 1.5 half-lives.
- 120 × 0.5^1.5 ≈ 42.43 mg.
Reading the result
The headline figure is the main answer. Any breakdown underneath shows the parts that make it up, so you can check the working and see what changes when you adjust an input.
Limitations and assumptions
- One dose at a time; add multiple drinks by running it for each and adding the results.
- Ignores absorption time.
- Not a medical assessment.
- For general education only — not medical advice. Caffeine half-life varies widely between people and with pregnancy, smoking, liver function and some medicines. Ask a health professional about your own limits.
- Inputs outside the supported range are rejected rather than forced into a result.
Reference: NCBI Bookshelf
Last reviewed:
Common questions
What half-life should I use?
Studies of healthy adults report averages of roughly 3 to 7 hours, and individuals vary more. Enter a value you understand applies to you, or try a range.
How much caffeine is in my drink?
Use the amount on the label or the maker's website; it varies a lot between brands and sizes.
Is there a safe amount at bedtime?
This calculator does not set a threshold. Sensitivity differs between people; ask a health professional for advice about your sleep.
How do I use the Caffeine Half-Life Calculator?
Enter the required values for Caffeine consumed, Time you had it, Bedtime, Half-life to assume. The calculator applies the formula on this page and shows the main result with any supporting breakdown so you can check the arithmetic.
What formula does the Caffeine Half-Life Calculator use?
Remaining = dose × 0.5^(hours elapsed ÷ half-life). This is first-order (exponential) elimination. The visible formula section above lists the calculation path and the edge cases the page handles, so the result can be checked without relying on the form alone.
Can the Caffeine Half-Life Calculator be used for exact decisions?
Use it as a calculation aid, not as a substitute for checking the underlying rule, contract, policy, or professional advice that applies to your situation. When a result depends on local rules, personal details, prices, or dates, enter those values yourself and confirm them before acting.
Sources
Method based on NCBI Bookshelf.
Cite this page
Free Calculator. (2026, November 3). Caffeine Half-Life Calculator. https://freecalculator.tech/tools/caffeine-half-life-calculator
"Caffeine Half-Life Calculator." Free Calculator, 3 Nov. 2026, https://freecalculator.tech/tools/caffeine-half-life-calculator.
Caffeine Half-Life Calculator — Free Calculator, https://freecalculator.tech/tools/caffeine-half-life-calculator
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