acalculator

How much caffeine is in my body?

Add each drink’s caffeine and how long ago you had it. The caffeine calculator shows your total against 400 mg and how much is left in your body.

Your numbers

Drinks today
Row 1
Row 2
More options
Caffeine in your body now
119

Of the 221 mg you had today, about 119 mg is still in your body.

Caffeine todaymg
221
Share of 400 mg
55.3%
Left before 400 mgmg
179
Against the FDA guide
Under the FDA’s 400 mg a day for healthy adults
Caffeine left latermg
39
Hours until half of now is gonehours
5

Caffeine in your body now: 119. Of the 221 mg you had today, about 119 mg is still in your body.

How does the caffeine wear off?

This calculator gives general information only. It is not medical advice and does not replace a doctor or other health care provider. Talk with a professional about your results. Terms of use

How to calculate

Adds up the caffeine you had today against the FDA’s 400 mg a day, and estimates how much is still in your body now and later from a 5-hour half-life.

Example with the default inputs (Drinks today [Caffeine (mg) 150, Hours ago 6; Caffeine (mg) 71, Hours ago 2], Half-life (hours) 5, Hours from now 8): Of the 221 mg you had today, about 119 mg is still in your body.

Method: Caffeine left = Σ mg × 0.5^((hours ago + hours from now) ÷ half-life); caffeine today = Σ mg, compared with 400 mg.

  • Each drink counts in full the moment you have it; in reality caffeine is absorbed over about an hour.
  • Caffeine leaves at a steady half-life, about 5 hours in adults. It is shorter in smokers and much longer in pregnancy and with some medicines.
  • The FDA’s 400 mg a day is for healthy adults; it is not a target, and some people should have less.
  • This is an estimate, not medical advice.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Drinks today 200 5, Half-life (hours) 5, Hours from now 5 gives Caffeine in your body now 100, Caffeine today 200, Share of 400 mg 50%, Left before 400 mg 200, Caffeine left later 50, Against the FDA guide Under the FDA’s 400 mg a day for healthy adults.Source: Shorter in smokers, 15 hours or more in late pregnancy (https://pubchem.ncbi.nlm.nih.gov/compound/Caffeine#section=Biological-Half-Life, retrieved 2026-10-02); FDA, Spilling the Beans: How Much Caffeine is Too Much?: 400 mg a day for healthy adults; caffeine per 12 fl oz of drinks (https://www.fda.gov/consumers/consumer-updates/spilling-beans-how-much-caffeine-too-much, retrieved 2026-10-02)
  2. Drinks today 150 6; 71 2, Half-life (hours) 5, Hours from now 8 gives Caffeine today 221, Share of 400 mg 55.3%, Caffeine in your body now 119.09923, Caffeine left later 39.288094.Source: Shorter in smokers, 15 hours or more in late pregnancy (https://pubchem.ncbi.nlm.nih.gov/compound/Caffeine#section=Biological-Half-Life, retrieved 2026-10-02); FDA, Spilling the Beans: How Much Caffeine is Too Much?: 400 mg a day for healthy adults; caffeine per 12 fl oz of drinks (https://www.fda.gov/consumers/consumer-updates/spilling-beans-how-much-caffeine-too-much, retrieved 2026-10-02)
  3. Drinks today 247 3; 246 1, Half-life (hours) 5, Hours from now 0 gives Caffeine today 493, Share of 400 mg 123.3%, Left before 400 mg 0, Against the FDA guide Over the FDA’s 400 mg a day for healthy adults.Source: FDA, Spilling the Beans: How Much Caffeine is Too Much?: 400 mg a day for healthy adults; caffeine per 12 fl oz of drinks (https://www.fda.gov/consumers/consumer-updates/spilling-beans-how-much-caffeine-too-much, retrieved 2026-10-02): up to 247 mg in 12 fl oz of brewed coffee and up to 246 mg in an energy drink

How it works

Type 1 to 20 drinks, each with its caffeine in milligrams (0 to 2,000) and how many hours ago you had it (0 to 24). A row with an empty box gives no answer, with a message naming the row.

  • Caffeine today = the sum of the drinks’ milligrams, worked out exactly from the typed decimals and shown to at most 6 decimal places, rounded half up.
  • Share of 400 mg = caffeine today ÷ 400 × 100%, rounded half up to 0.1%.
  • Left before 400 mg = 400 − caffeine today (exact, shown the same way), or 0 when caffeine today is 400 mg or more.
  • Against the FDA guide: under, exactly at, or over 400 mg.
  • Caffeine in your body now = Σ mg × 0.5^(hours ago ÷ half-life), shown to the whole milligram.
  • Caffeine left later = Σ mg × 0.5^((hours ago + hours from now) ÷ half-life), with no more drinks, shown to the whole milligram. Hours from now is 0 to 48 (default 8).
  • Hours until half of now is gone = the half-life (default 5 hours, from 1 to 30).

The chart draws the caffeine left over the next 24 hours, marked at your hours from now.

Assumptions

  • Each drink counts in full the moment you have it (no absorption time), then halves every half-life (first-order elimination).
  • The 400 mg guide is for healthy adults, not for children, pregnancy or people with health conditions.

Worked examples by hand

One 200 mg coffee, 5 hours ago, half-life 5 hours. Now: 200 × 0.5^(5 ÷ 5) = 100 mg. 5 hours from now: 200 × 0.5^(10 ÷ 5) = 50 mg. Today: 200 mg, which is 200 ÷ 400 = 50% of the guide, with 200 mg left before 400 mg: under the guide.

A 150 mg coffee 6 hours ago and a 71 mg black tea 2 hours ago. Today: 150 + 71 = 221 mg, 221 ÷ 400 = 55.25%, which rounds half up to 55.3%. Now: 150 × 0.5^1.2 + 71 × 0.5^0.4 = 65.29 + 53.81 = 119 mg. 8 hours from now: 150 × 0.5^2.8 + 71 × 0.5^2 = 21.54 + 17.75 = 39 mg.

A 247 mg coffee and a 246 mg energy drink. Today: 493 mg, 123.25%, shown as 123.3%: over the FDA’s 400 mg, with 0 mg left.

Other questions people ask

How much caffeine is too much?

For healthy adults, the FDA cites 400 mg a day, about two to three 12-ounce cups of coffee, as an amount not generally linked to negative effects. People differ, and pregnant people and those with some health conditions or medicines may need less. Ask a doctor.

How much caffeine is in coffee, tea and soda?

Per 12 fl oz, the FDA lists 113 to 247 mg for brewed coffee, 71 mg for black tea, 37 mg for green tea, 23 to 83 mg for caffeinated soft drinks and 41 to 246 mg for energy drinks. Check the label for your drink.

What is the half-life of caffeine?

About 5 hours in an average adult (PubChem): after 5 hours half is left, after 10 hours a quarter. It is shorter in smokers and can be 15 hours or more late in pregnancy. Change the half-life under More options.

How long does caffeine stay in your body?

It halves every half-life, so it never reaches exactly 0. A 200 mg coffee at a 5-hour half-life leaves 100 mg after 5 hours, 50 mg after 10 and 25 mg after 15 hours.

When should I stop drinking coffee before bed?

Set Hours from now to your bedtime and see what is left then. A 150 mg coffee 6 hours before the check and a 71 mg tea 2 hours before leave about 119 mg now and 39 mg eight hours later.

Does this count the time to absorb caffeine?

No. It counts each drink in full from the moment you have it. Caffeine reaches its peak in the blood a little later, so the real amount soon after a drink is a bit lower.