How to use
This calculator covers today only. Choose Current time to use the moment you press Calculate, or choose another time today. The result is a snapshot; calculate again to refresh it. Times use your device’s time zone, shown in Options & reference.
For each intake, select a drink or Enter mg directly in the same dropdown, then enter its time and amount. A drink serving means the size printed in the dropdown; use 0.5 for half or 2 for two taken together. The estimated caffeine appears below the drink. Switching between a drink and direct mg keeps the values you entered for each method.
Add a row for each different intake time. Use × to remove a row and Undo removal to restore the last deleted row. The result starts with the total remaining and a short summary. Open Details & chart for each intake’s contribution, the graph, time table and assumptions. Copy result includes that full detail even when it is collapsed. Edits invalidate the result; Clear starts a new list.
The model and its assumptions
For each intake, Rᵢ(T) = Dᵢ × 2^(−(T−tᵢ)/H), where Dᵢ is caffeine in milligrams, tᵢ is the intake time, T is the time being examined and H is the assumed half-life in hours. The combined result is the sum of those remaining amounts. Each dose starts decaying at its own intake time; adding the original doses and applying the time of the first drink would give the wrong answer.
The calculation uses an immediate starting dose and constant fractional elimination. It does not simulate absorption, blood concentration or individual physiology. The four-hour default is an illustrative assumption based on EFSA’s 2015 explanation, which reports an average of about four hours and a range of two to eight hours in healthy adults. You can change the assumption under Options & reference. The allowed 0.5–24-hour range is a technical limit, not a measured population range.
Only entered intakes are included. Drink presets use approximate generic serving examples, not brand-specific measurements. Use direct milligrams when you have reliable information for your actual portion. Neither the curve nor its remaining amount establishes a safe bedtime, driving ability, medication compatibility or an amount you may consume.
Two worked examples
With a four-hour half-life, 100 mg at 09:00 leaves 25 mg at 17:00 the same day. Add a separate 80 mg intake at 13:00: it leaves 40 mg at 17:00. The combined remaining amount is therefore 65 mg, from 180 mg entered. The intake breakdown displays 25 mg and 40 mg separately.
Two reference portions of filter coffee mean 400 mL and about 180 mg. If both are taken at 09:00, a four-hour half-life gives about 45 mg remaining at 17:00 today. Half a reference portion is 100 mL and about 45 mg initially. These figures assume the same drink and strength; an actual café portion may contain a different amount.
Reading the curve and table
The horizontal axis measures elapsed hours since the earliest entered intake; the table also shows local clock times. The curve rises at each intake and then decays until the next one. Values at an intake time include that intake. Before that time, it contributes nothing. Intakes at identical times are added together. The marked point is the selected calculation time.
The chart runs from the first entered intake to the end of today, labelled 24:00. Points after the time you are checking are projections assuming no additional intake. The table includes each intake time, the chosen time and regular intervals. The chart, table, breakdown and copied result use the same unrounded model.
Input limits, time and precision
Enter 1–20 intakes from today. An intake later than the selected calculation time is rejected rather than silently ignored. Each direct dose may be 0–10,000 mg; drink counts must be greater than zero and at most 20 reference servings per entry. These are technical limits, not consumption recommendations. Decimal points and commas are accepted for amounts without thousands separators or units.
Only today’s entered intakes are included; any caffeine remaining from earlier days is outside this calculation. Current time is captured to the minute when you calculate. The result displays its date and time so you can tell when a saved or copied result was calculated. All times follow the device’s local time zone. If daylight saving repeats a clock time, a manually entered time uses its earlier occurrence; this simple interface has no selector for the second occurrence.
Amounts use up to two decimal places; a positive amount below 0.01 mg displays as less than 0.01 mg. This is display precision, not measured clearance. Zero milligrams is valid and produces no remaining caffeine for that entry. Calculations stay in the browser. The page does not store a health history, and reloading clears the entered list.
Source for the default assumption
Frequently asked questions
Does 4 hours mean all caffeine is gone after 4 hours?
No. One half-life leaves half of the starting amount in this model. Two half-lives leave one quarter, and three leave one eighth.
Can I use this to choose a safe bedtime?
No. Remaining milligrams do not establish an individual's sleep response or a safe threshold. This page deliberately provides no bedtime verdict.
Can I combine drinks taken at different times?
Yes. Add a separate intake for each time, using either drink servings or direct milligrams. Each dose decays from its own time and the remaining amounts are summed. Only entered intakes are included.
Why can a tiny amount still appear after a long time?
The model decays continuously rather than stopping at a fixed cutoff. A less-than label reflects display precision, not a measured clearance time.