What is caffeine half-life?

Half-life is the time it takes for your body to eliminate half the caffeine in your bloodstream. With a 5-hour half-life, 200 mg at 9 AM becomes 100 mg by 2 PM, 50 mg by 7 PM, and 25 mg by midnight.

Why does the half-life vary between people?

The enzyme CYP1A2 in the liver metabolises caffeine. Its activity is strongly influenced by genetics (fast vs slow metaboliser alleles), hormonal contraceptives and pregnancy (which roughly double half-life), smoking (which nearly halves it), and certain medications that compete for the same pathway.

How much caffeine is safe in a day?

The European Food Safety Authority and most national health bodies consider up to 400 mg per day safe for healthy adults. Pregnant women are usually advised to stay under 200 mg. Sensitivity varies widely — some people feel anxious above 100 mg while others tolerate 600 mg without obvious symptoms.

How does caffeine affect sleep?

Caffeine blocks adenosine receptors. Adenosine is the molecule that makes you feel sleepy as it accumulates during the day. With receptors blocked, drowsiness is suppressed — but adenosine keeps accumulating. When caffeine clears, that backed-up adenosine floods in, which is why the post-caffeine crash can feel worse than if you had never had coffee.

What is the caffeine content of common drinks?

A single espresso averages about 63 mg, a 240 ml filter coffee around 95 mg, a 250 ml energy drink 80 mg, and black tea 47 mg. Cold brew is notably high at roughly 200 mg per 240 ml. These figures are averages — actual levels vary with roast, grind size, brew time and brand.

Is this tool accurate for my exact body?

The calculator uses the standard first-order exponential decay model (the same used in pharmacokinetics research) with your chosen half-life. Caffeine absorption is fairly complete within 30–60 minutes of consumption, which the model assumes. Individual variation in absorption speed, body composition, and enzyme activity means the result is a well-informed estimate, not a clinical measurement.

Caffeine Half-Life Calculator

Whether you are timing a pre-workout dose before training, wondering why you cannot fall asleep at midnight, or trying to schedule your last coffee of the day, this caffeine half-life calculator gives you a precise, evidence-based picture of exactly how much caffeine remains in your body at any point in time.

How the caffeine decay formula works

Caffeine follows first-order pharmacokinetics — meaning a fixed fraction of whatever is in your bloodstream is eliminated per unit of time. That produces exponential decay, described by:

C(t) = C₀ × 2^(−t ÷ half-life)

where C₀ is the initial dose in milligrams, t is the time elapsed since consumption in hours, and half-life is your personal clearance rate (default 5 h). The calculator applies this formula to every dose you enter, then sums the contributions to get your total current load.

To find when you hit a target (your sleep threshold), the formula is inverted:

t = half-life × log₂(C₀ ÷ threshold)

All arithmetic runs locally in your browser — nothing is sent to a server.

Why the half-life matters so much

With a 5-hour half-life a single 200 mg dose (two shots of espresso) decays like this:

Time after drinking	Caffeine remaining
0 h (just consumed)	200 mg
5 h	100 mg
10 h	50 mg
15 h	25 mg
20 h	12.5 mg

If you consumed those same two shots at 8 AM, you would still have roughly 50 mg circulating at 6 PM — enough to noticeably reduce sleep quality. Add a 3 PM coffee and you could easily cross 100 mg at midnight.

Worked example

Scenario: You have a double espresso (126 mg) at 8:00 AM and a filter coffee (95 mg) at 1:00 PM. Your half-life is 5 hours and you want to be below 25 mg before sleeping.

At 6:00 PM (10 h after dose 1, 5 h after dose 2):
- Espresso remaining: 126 × 2^(−10/5) = 126 × 0.25 = 31.5 mg
- Filter coffee remaining: 95 × 2^(−5/5) = 95 × 0.5 = 47.5 mg
- Total at 6 PM: 79 mg
Time from 6 PM until total drops to 25 mg:
- 5 × log₂(79/25) = 5 × 1.66 = 8.3 hours
- Safe to sleep from approximately 02:18 AM

Enter those numbers into the calculator above and the decay chart will confirm this with a visual curve.

Using your personal half-life

The population median is 5 hours but the range in healthy adults spans 1.5–9.5 hours. People who smoke metabolise caffeine roughly twice as fast (half-life ~3 h). Those taking hormonal contraceptives or who are pregnant may process it at half the speed (half-life ~9 h). If you know from experience that coffee keeps you wired for much longer or shorter than friends, shift the slider — the sleep-safe time will update immediately.