Last updated: September 24, 2026
What a caffeine half-life actually measures
Caffeine is cleared mainly by liver enzymes, and the amount removed in any given hour depends on how much is present at the time. A larger amount is removed faster in absolute terms, so the decline follows an exponential curve instead of a straight line. The half-life is the time it takes for the amount in the body to fall by half.
That makes a half-life a description of a rate, not a deadline. It says nothing about how alert you feel and nothing about how well you will sleep. It only describes how the amount is expected to shrink over time.
The five-hour default used here
Averages reported for healthy adults are commonly placed between roughly three and seven hours, and five hours is a convenient midpoint. CaffCutoff defaults to a five-hour half-life for every dose, which keeps estimates readable and comparable from one day to the next.
It is a planning assumption, not a measurement of you. Real half-lives shift with smoking, pregnancy, medications, liver function, and genetics, and the factors guide covers the direction of those shifts. If you want to see how much work that assumption is doing, the calculator's fast and slow presets reframe the same day on a four-hour and a seven-hour half-life.
Hour by hour at a five-hour half-life
Each row shows how much of a dose is still expected to remain after that many hours, applied to two common amounts. A 95 mg coffee and a 150 mg pre-workout are used only as illustrations of the curve.
| Hours after the dose | Share remaining | 95 mg coffee | 150 mg pre-workout |
|---|---|---|---|
| 0 h | 100% | 95 mg | 150 mg |
| 1 h | 87% | 83 mg | 131 mg |
| 2 h | 76% | 72 mg | 114 mg |
| 3 h | 66% | 63 mg | 99 mg |
| 4 h | 57% | 55 mg | 86 mg |
| 5 h | 50% | 48 mg | 75 mg |
| 6 h | 44% | 41 mg | 65 mg |
| 8 h | 33% | 31 mg | 49 mg |
| 10 h | 25% | 24 mg | 38 mg |
| 12 h | 19% | 18 mg | 28 mg |
| 15 h | 13% | 12 mg | 19 mg |
| 24 h | 4% | 3 mg | 5 mg |
Amounts are rounded, and the table assumes a five-hour half-life with no other adjustment. The same formula drives the calculator.
Half-life is not the same as time to zero
Because the amount keeps halving, it approaches zero without ever formally arriving there. Reporting "the exact time caffeine leaves your body" would therefore be misleading, so the calculator works with thresholds instead:
- 25 mg is the bedtime planning target built into this tool. It marks a point where the estimate has fallen far enough that a late drink is unlikely to add much.
- 5 mg is the clearance target used for the "below 5 mg" estimate on the report page.
Both are product planning values chosen so the output stays readable. Neither is a clinical threshold, an exposure limit, or medical advice.
A quick rule of thumb
Every half-life removes half of what remains: half after one, a quarter after two, an eighth after three, and roughly 3% after five. At a five-hour half-life, a single dose needs about 25 hours to fall under 3% of its original size. The final several hours of that window contain very little caffeine, which is why a cutoff estimate tends to be more useful than a "fully cleared" timestamp.
Two people, one drink, two very different curves
Because a half-life is a rate, small differences compound. A person clearing caffeine every three hours has far less left at bedtime than someone on a seven-hour curve, even if they drank the same cup at the same time. That is why the site keeps one clearly stated default instead of pretending to know your personal number.
Where this fits in the calculator
The calculator applies the same decay to every drink you add, then sums the remainders to estimate the caffeine left at bedtime. From there it derives a caffeine cutoff time for your next drink. If you need representative amounts to enter, the caffeine content chart lists common sources.
Common questions
Is a five-hour caffeine half-life accurate for everyone?
No. Five hours is a widely used midpoint for healthy adults, but commonly reported averages run from roughly three to seven hours and can be longer during pregnancy or with certain medications. Treat the result as a planning estimate.
Does a longer half-life mean I am more sensitive to caffeine?
Not exactly. Half-life describes how quickly caffeine is cleared from the body, while sensitivity describes how strongly you respond to the caffeine that is present. The two often overlap, but they are separate ideas.
Does caffeine ever fully leave the body?
Under a half-life model the amount keeps halving and approaches zero without formally reaching it. CaffCutoff therefore reports thresholds instead, such as the 25 mg bedtime planning target and the 5 mg clearance target.
Why does the calculator estimate each drink separately?
Each dose is cleared independently, so its own clock starts when you finish it. Estimating each dose separately and then adding the remainders reflects that better than averaging the whole day into one number.
Related reading
- Caffeine Metabolism: How Your Body Clears Caffeine
- Caffeine Content Chart for Common Drinks
- Caffeine Cutoff Time: When Should You Stop Drinking Coffee?
- Coffee Before Bed: How Late Is Too Late to Drink Coffee?
- What Changes Caffeine Half-Life?
- Am I a Slow Caffeine Metabolizer? A Self-Check
- How Much Caffeine Is Left? Dose by Hour Chart
- How Much Caffeine Is Left at Bedtime?
Run your own numbers
The CaffCutoff calculator turns these ideas into an estimate for your own day: add each drink, set a target bedtime, and read the remaining caffeine, the cutoff estimate, and the decay timeline.