# How much caffeine is in my body?

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.

- Page: https://www.acalculator.org/health/caffeine-calculator
- JSON spec: https://www.acalculator.org/health/caffeine-calculator.json
- Version: 779a4a1ae33a

## Default answer

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.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| d | Drinks today | One row per drink: its caffeine in milligrams and how many hours ago you had it. |
| half | Half-life (hours) | The hours for your body to clear half the caffeine: about 5 for adults (PubChem); longer in pregnancy. |
| later | Hours from now | A time to check, for example your bedtime, in hours from now. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| now | Caffeine in your body now | What is left of every drink after its hours ago, halving every half-life. |
| total | Caffeine today | All the caffeine of the drinks added up. |
| share | Share of 400 mg | Caffeine today as a percent of the FDA’s 400 mg a day for healthy adults. |
| room | Left before 400 mg | 400 mg minus the caffeine today, or 0 when you are over. |
| limit | Against the FDA guide | Whether today’s caffeine is under, at or over 400 mg. |
| laterMg | Caffeine left later | What will be left at the hours from now you chose, with no more drinks. |
| halfLife | Hours until half of now is gone | One half-life: the time for the caffeine in your body now to halve. |

## Method

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

## Assumptions

- 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.

## Worked examples

1. d = {"mg":200,"ago":5} or undefined, half = 5, later = 5 gives now = 100, total = 200, share = 50%, room = 200, laterMg = 50, limit = 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. d = {"mg":150,"ago":6} or {"mg":71,"ago":2}, half = 5, later = 8 gives total = 221, share = 55.3%, now = 119.09923, laterMg = 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. d = {"mg":247,"ago":3} or {"mg":246,"ago":1}, half = 5, later = 0 gives total = 493, share = 123.3%, room = 0, limit = 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.

## FAQ

### 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.

## Sources

- U.S. Food and Drug Administration, Spilling the Beans: How Much Caffeine is Too Much? (400 mg a day for healthy adults; caffeine per 12 fl oz: soft drinks 23–83 mg, green tea 37 mg, black tea 71 mg, brewed coffee 113–247 mg, energy drinks 41–246 mg), retrieved 2026-10-02. https://www.fda.gov/consumers/consumer-updates/spilling-beans-how-much-caffeine-too-much
- National Library of Medicine, PubChem, Caffeine (CID 2519), Biological Half-Life (about 5 hours in an average adult; up to 50% shorter in smokers; 15 hours or more in pregnancy), retrieved 2026-10-02. https://pubchem.ncbi.nlm.nih.gov/compound/Caffeine#section=Biological-Half-Life