Caffeine Half-Life Calculator

See how long caffeine stays in your system. This free caffeine half-life calculator estimates how much remains now and at bedtime from your dose, when you had it, and your metabolism.

This is a general, educational estimate of how caffeine clears from the body. Everyone metabolises caffeine differently — pregnancy, medications, liver health and genetics all change the rate. For healthy adults, up to about 400 mg of caffeine a day is generally considered moderate (less in pregnancy); if caffeine affects your sleep, heart or anxiety, speak with a doctor.
Your caffeine
mg
hours ago
h

Enter how much caffeine you had and how long ago. Pick the metabolism that fits you. A common reference for a coffee is about 95 mg; an energy drink ~80 mg; a tea ~40 mg.

In your system now
—
Enter your caffeine
% of dose left
—
At bedtime
—
Down to half
—
Under ~50 mg in
—

Caffeine decays exponentially: remaining = dose × ½^(hours ÷ half-life). With an average half-life of about 5.7 hours, roughly half of a dose remains after ~6 hours and a quarter after ~12 — which is why afternoon coffee can still affect sleep.

Use this free caffeine half-life calculator to see how much caffeine is still in your system. Enter a dose and when you had it to estimate what’s left now and at bedtime.

What this caffeine half-life calculator does

A caffeine half-life calculator models how caffeine clears from your body over time. Caffeine decays exponentially, so the calculator shows the amount remaining now, at bedtime, and when it drops to a sleep-friendly level.

The half-life formula

remaining = dose × ½^(hours ÷ half-life)
average caffeine half-life ≈ 5.7 hours

So 200 mg of caffeine falls to about 100 mg after ~6 hours and 50 mg after ~12 hours — which is why an afternoon coffee can still affect sleep.

Caffeine in common drinks

Drink Typical caffeine
Brewed coffee (8 oz) ~95 mg
Espresso (1 shot) ~63 mg
Energy drink (8 oz) ~80 mg
Black tea ~40 mg
Cola (12 oz) ~35 mg

What changes your caffeine half-life

The 5.7-hour average is just that — an average. Pregnancy can roughly double it; some medications and liver conditions slow it; smoking speeds it up. Genetics make some people fast and others slow metabolisers.

How to use the caffeine calculator

  1. Enter the dose and how many hours ago you had it.
  2. Pick your metabolism (fast, average or slow).
  3. Add hours until bedtime to see what’s left when you sleep.

Caffeine Half-Life FAQ

What is the half-life of caffeine?

About 5.7 hours on average for healthy adults, meaning half the caffeine is cleared in that time. It varies with genetics, pregnancy, medications and liver health.

How long does caffeine stay in your system?

Most is gone within about 10–12 hours, but a meaningful amount can linger. After 200 mg, roughly 50 mg may still remain around 12 hours later.

How is caffeine clearance calculated?

Using exponential decay: remaining = dose × ½^(hours ÷ half-life). The calculator applies the half-life you select.

Why does afternoon coffee affect sleep?

Because half-life is several hours, caffeine from a mid-afternoon coffee can still be active at bedtime, making it harder to fall or stay asleep.

How much caffeine is safe per day?

Up to about 400 mg a day is generally considered moderate for healthy adults, and less during pregnancy. Tolerance varies, so adjust to how it affects you.

Is the caffeine half-life calculator free?

Yes, this caffeine half-life calculator is completely free, needs no sign-up, and gives instant results in your browser.

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