Caffeine Half-Life Calculator

Last updated: 2026-05-12

Caffeine Clearance from the Body

Caffeine is eliminated from the body through exponential decay: at a typical half-life of 5–6 hours, roughly half of each dose is cleared every 5–6 hours. Enter your intake amount, the time you drank it, and a check time — the calculator shows how many milligrams remain and what fraction of the original dose is still active.

This matters most for sleep timing: a dose consumed in the afternoon may still be pharmacologically significant at bedtime, even when it no longer feels stimulating.

The Formula

Caffeine follows first-order pharmacokinetics — a constant fraction is eliminated per unit time, regardless of the current concentration:

$C(t) = C_0 \times 0.5^{\, t \,/\, t_{1/2}}$

Where:

• $C_0$ = initial dose (mg)
• $t$ = elapsed time since intake (hours)
• $t_{1/2}$ = half-life (hours, default 5.7 h)
• $C(t)$ = caffeine remaining at time $t$

After one half-life, 50% remains. After two, 25%. After three, 12.5%. The process is exponential, so caffeine concentration never reaches exactly zero — but it falls to levels too small to matter within about 5 half-lives (roughly 28 h at the 5.7 h default).

Half-Life: Definition

Half-life is a concept borrowed from nuclear physics and applied to pharmacology: the time it takes for the concentration of a substance to halve in the body. Unlike a linear model (where the same absolute amount is eliminated each hour), exponential decay means each successive half-life eliminates less caffeine in absolute terms — but always the same fraction.

This matters in practice because it means large doses linger proportionally longer. A 400 mg intake (four strong coffees) after one half-life becomes 200 mg — still a significant stimulant dose — while a 100 mg intake becomes only 50 mg.

Caffeine Half-Life: Individual Variation

The default of 5.7 hours is a population average for healthy, non-smoking adults. But half-life varies considerably:

The primary enzyme responsible for caffeine metabolism is CYP1A2, a liver enzyme whose activity varies genetically and is modulated by many drugs and lifestyle factors. Smokers have high CYP1A2 activity (and fast caffeine clearance); pregnancy dramatically suppresses it.

If you are a fast metabolizer (perhaps a genetic variant), half-life can be as low as 2–3 hours. If you are slow, it can approach 10 hours even without any of the pharmacological modifiers above.

Caffeine Thresholds for Sleep Disruption

Sleep research consistently finds that caffeine within 6 hours of bedtime delays sleep onset and reduces slow-wave sleep quality — even when subjects do not feel subjectively more alert. A 2013 study by Drake et al. in the Journal of Clinical Sleep Medicine found measurable sleep disruption from caffeine consumed as early as 6 hours before bedtime.

A rough practical guideline from this literature: aim for less than 25% of your dose remaining at bedtime. At a 5.7 h half-life, that corresponds to roughly 2 half-lives elapsed, or about 11.4 hours of clearance time. For a 95 mg intake (one coffee), 25% = 23.75 mg — well below the threshold most people find stimulating.

Use the calculator to find the "safe intake window" for your personal half-life and bedtime: set the check time to your target sleep time, adjust the half-life to match your sensitivity, and work backward to find when your intake should stop.

Practical Examples

The 3 pm Coffee Problem

You drink a 200 mg coffee at 15:00 and plan to sleep at 23:00. With a 5.7 h half-life:

$C(8) = 200 \times 0.5^{8/5.7} = 200 \times 0.5^{1.40} \approx 200 \times 0.378 = 75.6 \text{ mg}$

75 mg is still a meaningful stimulant dose. If sleep quality matters to you, a 15:00 cut-off with a 5.7 h half-life and a 23:00 bedtime is borderline.

The Afternoon Nap Strategy

The "caffeine nap" (coffee immediately followed by a 20-minute nap) is sometimes used to time the caffeine peak coinciding with the end of the nap. Caffeine takes roughly 30–45 minutes to peak in the bloodstream, so the nap occupies the absorption window. This is a legitimate sleep-hack — though the half-life still applies from that peak.

Long-Half-Life Scenarios

If you are on oral contraceptives (half-life ~8 h) and drink 200 mg at 08:00, checking at 22:00:

$C(14) = 200 \times 0.5^{14/8} = 200 \times 0.5^{1.75} \approx 200 \times 0.297 = 59.4 \text{ mg}$

Compare this to the same scenario with a 5.7 h half-life: ≈ 37 mg at 22:00. Pharmacological context matters significantly.

The Next-Morning Check

A late-night cup raises an obvious question: how much is left when you wake up? Say you drink a 150 mg coffee at 22:00 and want to know the level at 07:00 the next morning. Because the check time is on the following calendar day, turn on the Next day toggle — the calculator then counts 9 hours of elapsed time (2 hours to midnight plus 7 after), not the −15 hour clock difference. With a 5.7 h half-life:

$C(9) = 150 \times 0.5^{9/5.7} = 150 \times 0.5^{1.58} \approx 150 \times 0.334 = 50.1 \text{ mg}$

Leave the toggle on Same day whenever intake and check fall within one calendar day.

Inputs and Defaults

Common caffeine doses:

• 8 oz brewed coffee: 80–100 mg (≈ 95 mg average)
• Single espresso shot: 63 mg
• Double espresso: 125 mg
• Black tea (8 oz): 47 mg
• Green tea (8 oz): 28 mg
• Cola (12 oz): 34 mg
• Energy drink: 80–300 mg (highly variable)
• Dark chocolate (1 oz): 12 mg

Limitations

This model assumes:

1. Instantaneous absorption. In reality, caffeine peaks in plasma 30–45 minutes after oral ingestion. For check times very soon after intake, the actual concentration may be lower than predicted.
2. Single-dose model. If you drink multiple coffees, the model does not add them. You would need to run separate calculations and sum the results.
3. Population average half-life. The 5.7 h default does not account for individual genetics, health status, or medications. Adjust the half-life field to better reflect the relevant case.
4. No tolerance effects. Regular caffeine consumers develop adenosine receptor upregulation, changing subjective sensitivity without changing pharmacokinetics.

For sleep-related planning, see Sleep Cycle Calculator (optimal wake times from sleep cycles) and Daily Water Intake Calculator (daily hydration needs).