Caffeine Calculator

Caffeine Calculator: What remains when you go to bed?

The Caffeine Calculator estimates how much caffeine remains after a coffee. Enter the starting dose in milligrams and elapsed hours, optionally adding hours until bedtime. Use the result to understand your afternoon drink, not as a sleep guarantee.

How it works

The model uses exponential decay: remaining amount = starting amount × 0.5 raised to (elapsed hours / half-life). Leaving the half-life blank selects five hours. For example, 200 mg becomes 100 mg after five hours and 50 mg after ten hours. To estimate the amount at bedtime, the calculator adds elapsed hours to the hours still ahead.

The table follows the decline from now. The reverse formula, half-life × log₂(starting amount / 50 mg), finds the required gap before your target time. Doses already below the target need no waiting time. The chosen 50 mg is not a medical sleep threshold.

What it can and cannot do

Estimate one dose or drinks consumed together. Do not combine drinks spread throughout the day: each has a different decay time. This tool excludes blood measurements, delayed absorption and later drinks. A cup of filter coffee contains roughly 80 to 90 mg, an espresso about 60 mg, cola around 25 mg per 250 ml and black tea about 45 mg. Preparation and portion sizes change these estimates; check the label where available.

Note: For orientation only, not medical advice. Seek medical advice for symptoms or persistent sleep problems instead of relying on another calculation.

Why local processing in the browser

Your device handles the calculation without an account. No remote server needs your dose or bedtime. Once the page and script load, you can try more values offline. Reopening the page without a connection depends on which files your browser has kept.

Creative uses

• Compare the same coffee consumed earlier in the day.
• Check the gap before planned rest after a night shift.
• Try a longer half-life to see how the estimate changes.
• Compare coffee and tea using their estimated caffeine content.
• Add a calculated decay timeline to a sleep diary.

A small contribution to climate protection

Each calculation stays in the browser, avoiding another server request. The historical estimate of 1.76 g CO2 per page view gives context, not a measurement of this page. The IEA reports that data centres consumed 415 TWh in 2024. Local processing avoids repeated server round-trips, not all energy demand. These figures cannot establish carbon savings per calculation.

Frequently asked questions