Caffeine Clearance Calculator — When to Stop

Caffeine wears off slower than it feels like it does. Long after the alertness fades, a meaningful amount can still be circulating — quietly cutting into the deep sleep you can’t consciously feel it stealing. This calculator logs what you drank and when, then shows how much caffeine is still in your system at bedtime, a decay curve across your day, and a “last call” time to protect your sleep.

Add your drinks below. It takes about 30 seconds, and nothing is stored.

How long does caffeine stay in your system?

Caffeine has an average half-life of about 5 hours in healthy adults — meaning that’s how long it takes your body to clear half of a given dose. So a 200 mg coffee at 3 p.m. leaves roughly 100 mg at 8 p.m. and about 50 mg by 1 a.m. It clears through a first-order process: a fixed fraction leaves every hour, so the curve falls fast at first and then trails off.

That 5-hour figure is only an average, though. Real half-lives range from roughly 1.5 to 9.5 hours depending on your genetics, medications, and lifestyle — which is why the calculator lets you adjust it.

What changes how fast you clear caffeine?

Caffeine is broken down almost entirely by one liver enzyme (CYP1A2), and several things speed it up or slow it down:

• Smoking speeds it up — it roughly halves your half-life, so caffeine clears faster.
• Oral contraceptives slow it down — they can roughly double it, so a morning coffee lingers longer.
• Pregnancy slows it down substantially — half-life can stretch to 10 hours or more, especially later in pregnancy.
• Genetics — common CYP1A2 variants make some people “fast” and others “slow” metabolizers.

Set the option that best matches you for a more accurate curve. If you’re unsure, the average adult setting is a reasonable default.

How much caffeine is too much before bed?

Timing matters as much as amount. In a controlled study, a dose of caffeine taken even 6 hours before bedtime measurably reduced total sleep time — enough that researchers recommended avoiding meaningful caffeine for at least 6 hours before bed (Drake et al., 2013). Many sleep researchers treat roughly 50 mg still circulating at bedtime as a practical guide for sensitive sleepers, which is the marker this calculator uses.

The point isn’t to quit coffee — it’s to move it earlier. Your “last call” time shows when a standard coffee should be your final one so it falls below that guide by the time you’re trying to sleep.

How this calculator works

Each drink decays on its own exponential curve — the amount left equals the dose × ½ raised to (hours elapsed ÷ half-life) — and the calculator adds them together to plot your total caffeine over the day. Half-life defaults to the ~5-hour adult average and can be adjusted for smoking, oral contraceptives, pregnancy, or a custom value. Caffeine amounts per drink are approximate and vary by brand, bean, and serving size, so treat the curve as a well-informed estimate rather than a lab measurement.

Frequently Asked Questions