Caffeine Half-Life Calculator

About this calculator

This calculator estimates how much caffeine is still in your body at any point after a coffee, tea, energy drink, or pre-workout — and when it falls low enough that it is unlikely to keep you awake. Enter what you drank and how long until bedtime to see whether that afternoon cup is still working against your sleep.

How to read your results

The headline figure is the caffeine still active at your target bedtime. The decay curve plots the amount in your body over the next 24 hours, and the dashed line marks the 50 mg "sleep threshold" — a level low enough that most people can fall asleep normally. If the curve is above the line at bedtime, the result is flagged as "still wired"; the "below 50 mg in" stat tells you how many hours after your drink you reach a sleep-friendly level.

Worked example

One 95 mg brewed coffee, average metabolism (5-hour half-life), 6 hours before bed.

About 41 mg remains at bedtime — just under the 50 mg threshold, so it is flagged "clear for sleep." Caffeine drops below 50 mg about 4.6 hours after the cup and is effectively cleared (under 5 mg) after roughly 21 hours.

Frequently asked questions

What is caffeine half-life?

Half-life is the time it takes your body to eliminate half of the caffeine present. With an average half-life of about 5 hours, a 200 mg dose falls to 100 mg after 5 hours, 50 mg after 10 hours, and 25 mg after 15 hours. Because elimination is proportional rather than constant, caffeine lingers far longer than most people expect.

Why does the calculator use a 50 mg sleep threshold?

There is no universal cut-off, but research links caffeine near bedtime to longer time falling asleep and lighter sleep. 50 mg is a pragmatic "low" level — roughly half a cup of coffee — below which most people are unlikely to notice an effect. People who are caffeine-sensitive, pregnant, or taking certain medications should aim lower.

What makes someone a fast or slow metaboliser?

Caffeine is broken down mainly by the liver enzyme CYP1A2, and its activity varies with genetics, age, pregnancy, smoking, and some medications (for example, certain antibiotics and oral contraceptives slow it down). Slow metabolisers can have a half-life of 6 hours or more, so the same coffee keeps them awake longer. Pick the option that best matches your experience.

How much caffeine is safe in a day?

The FDA cites up to 400 mg per day — about four to five cups of coffee — as not generally associated with negative effects for healthy adults. Pregnant people are usually advised to stay under 200 mg. This tool estimates timing and sleep impact, not safety, and is not medical advice.

How it's calculated

Caffeine is eliminated by first-order kinetics: a constant fraction leaves the body per unit time, so the amount remaining halves every half-life. The calculator uses C(t) = C₀ × (½)^(t ÷ half-life), where C₀ is the dose consumed. A healthy adult half-life averages about 5 hours; the "fast / average / slow" options use 4, 5, and 6 hours to reflect normal variation. Time to reach a target level is t = half-life × log₂(C₀ ÷ target). The default per-serving caffeine amounts follow FDA and USDA figures for common drinks.