Caffeine Half-Life Calculator
Estimate how much caffeine may still be in your body now and at bedtime, then follow its decline on a personalised timeline.
Calculate caffeine remaining
Enter one drink or dose. All results are estimates.
Individual metabolism varies widely. Five hours is a modelling example, not a personal measurement.Estimated amount now
This cannot predict how alert you will feel or measure caffeine in your blood.
Your estimated caffeine timeline
How long does caffeine stay in your system? This caffeine half-life calculator estimates how much of a single dose may remain over time. Enter the caffeine amount, when you consumed it, your expected bedtime and a modelling half-life. The calculator then shows an estimate for now, bedtime and several points on an hourly timeline.
Caffeine metabolism differs considerably between people, so the result is not a blood measurement or a prediction of exactly when its effects will disappear. Genetics, pregnancy, medicines, smoking, liver health and other factors can change the rate at which caffeine is cleared.
How the caffeine half-life calculator works
A substance's half-life is the time required for its amount in the body to fall by half. The calculator uses exponential decay:
With a five-hour half-life, a 100 mg dose is modelled as approximately 50 mg after five hours, 25 mg after ten hours and 12.5 mg after fifteen hours. Elimination is gradual; caffeine does not suddenly disappear after one half-life.
Approximate caffeine in common drinks
Amounts differ by brand, serving size, beans and preparation method. Product labels are more reliable than generic estimates, particularly for energy drinks and supplements.
| Drink or product | Example serving | Approximate caffeine |
|---|---|---|
| Brewed coffee | One cup | About 80–120 mg |
| Espresso | Single shot | About 60–75 mg |
| Black tea | One cup | About 30–50 mg |
| Green tea | One cup | About 20–40 mg |
| Energy drink | 250 ml can | Often around 80 mg |
| Pre-workout supplement | One serving | Commonly 150–300 mg, but varies |
What changes caffeine metabolism?
Genetics and individual sensitivity
Liver enzymes clear most caffeine, and their activity varies. Two people drinking the same coffee at the same time can therefore have different amounts remaining later. Feeling accustomed to caffeine also does not prove that it has left the body.
Pregnancy
Caffeine clearance becomes slower during pregnancy, especially later pregnancy. Pregnant people should follow pregnancy-specific caffeine guidance rather than using a standard adult half-life as a safety calculation.
Medicines, smoking and health conditions
Some medicines and health conditions can slow caffeine clearance, while smoking can speed it. Do not alter medication or use this calculator to decide whether a caffeine dose is medically safe.
Caffeine and sleep
Caffeine promotes alertness partly by blocking adenosine receptors. A meaningful amount can remain around bedtime even when the strongest noticeable effect has faded. If sleep is difficult, recording caffeine amounts and times for a week may reveal whether an earlier cut-off helps.
Reducing caffeine gradually may be more comfortable for regular users because abrupt withdrawal can cause headache, tiredness and irritability. Decaffeinated drinks still contain small amounts but usually much less than standard coffee or tea.
Caffeine calculator FAQ
What is the average half-life of caffeine?
Five hours is commonly used as an adult example, but reported values vary. FDA material notes an average range of roughly two to six hours in healthy adults and says it can be much longer in some individuals.
How long until caffeine is completely gone?
There is no precise universal time. After five half-lives, about 3% of the original modelled dose remains. With a five-hour half-life that is around 25 hours, but actual metabolism varies.
How much caffeine is safe per day?
The FDA cites 400 mg per day as an amount not generally associated with negative effects for most adults. This is not appropriate for everyone, and pregnancy-specific limits are lower.
Can I flush caffeine out by drinking water?
Water can support normal hydration but does not instantly remove caffeine. The liver must metabolise it over time.
Why can I sleep after coffee?
People differ in tolerance, sensitivity and metabolism. Being able to fall asleep does not necessarily mean caffeine has left the body or has no effect on sleep quality.
Sources
Key references include the US Food and Drug Administration caffeine guidance and the European Food Safety Authority scientific opinion on caffeine safety.
Medical notice: This calculator is a simplified educational model. It cannot measure blood caffeine, determine fitness to drive, diagnose symptoms or establish a safe personal dose. Seek urgent medical help after a suspected caffeine overdose or for severe symptoms such as chest pain, collapse, confusion, seizures or a markedly abnormal heartbeat.