See how long caffeine stays in your system. This free caffeine half-life calculator estimates how much remains now and at bedtime from your dose, when you had it, and your metabolism.
Enter how much caffeine you had and how long ago. Pick the metabolism that fits you. A common reference for a coffee is about 95 mg; an energy drink ~80 mg; a tea ~40 mg.
Caffeine decays exponentially: remaining = dose x ½^(hours / half-life). With an average half-life of about 5.7 hours, roughly half of a dose remains after ~6 hours and a quarter after ~12 — which is why afternoon coffee can still affect sleep.
How the Caffeine Half-Life Calculator Works
This tool estimates how much caffeine is likely still active in your system based on how much you had, how long ago, and how quickly your body typically clears it. Caffeine doesn’t disappear in a straight line — it decays exponentially, so it drops fastest right after you drink it and more slowly as time goes on.
Formula: Caffeine remaining now = Dose × 0.5^(Hours ago ÷ Half-life). To estimate the amount at bedtime, the same formula is used with (Hours ago + Hours until bedtime) in place of Hours ago. The “under ~50 mg” estimate solves that formula in reverse for the number of hours from now until the remaining amount drops to about 50 mg.
- Dose — the amount of caffeine consumed, in milligrams (e.g., ~95 mg for a coffee, ~80 mg for an energy drink, ~40 mg for tea).
- Hours ago — how long ago you consumed it.
- Half-life — the metabolism speed you select: fast (~4 h), average (~5.7 h), slow (~7 h), or very slow (~9 h).
- Hours until bedtime — optional; used to estimate how much caffeine will still be active when you plan to sleep.
Example Scenarios
| Dose | Hours Ago | Metabolism (Half-Life) | Caffeine Remaining Now |
|---|---|---|---|
| 200 mg | 3 h | Average (5.7 h) | ~139 mg |
| 95 mg | 6 h | Average (5.7 h) | ~46 mg |
| 300 mg | 8 h | Fast (4 h) | 75 mg |
| 150 mg | 4 h | Slow (7 h) | ~101 mg |
| 80 mg | 10 h | Very slow (9 h) | ~37 mg |
| 400 mg | 12 h | Average (5.7 h) | ~93 mg |
Caffeine Half-Life Calculator FAQ
How long does caffeine actually take to leave your system completely?
Caffeine decays exponentially rather than stopping at a fixed time, so it never hits exactly zero — it just gets progressively smaller. With an average half-life of about 5.7 hours, only a small fraction typically remains after 24 hours, though genetics and other factors change this timeline person to person.Does “half-life” mean caffeine is gone after that many hours?
No — the half-life is the time for the amount in your body to drop by half, not to disappear. After one half-life roughly 50% remains, after two about 25%, and so on, which is why the decay curve flattens out rather than ending abruptly.Why does the same amount of coffee affect people so differently?
Caffeine metabolism speed varies with genetics (particularly the CYP1A2 liver enzyme), pregnancy, certain medications, liver function, and smoking status. That’s why this calculator offers a range of metabolism speeds instead of a single fixed rate for everyone.Does drinking coffee earlier in the day still affect nighttime sleep?
It can. Because clearance is gradual, an afternoon coffee can still leave a meaningful amount of caffeine active by bedtime, especially for slower metabolizers. The “at bedtime” estimate in this calculator is designed to show that carryover.Is caffeine cleared more slowly during pregnancy?
Many sources note that caffeine clearance slows considerably during pregnancy, meaning it can stay active in the body noticeably longer than usual. This calculator’s slower metabolism settings can be used as a rough approximation, but a doctor’s guidance on caffeine intake during pregnancy is more reliable than any general estimate.What’s a reasonable caffeine cutoff time before bed?
There’s no single universal cutoff since it depends on your dose, metabolism, and sensitivity, but many people find that stopping caffeine at least 6-8 hours before their target bedtime reduces how much is still active when they try to sleep. Use the “hours until bedtime” field to check your own numbers.Related Calculators
If you’re curious how your daily caffeine habit compares to general guidance, the caffeine safe limit calculator is a useful next stop. Since leftover caffeine at night is closely tied to how well you sleep, it’s also worth checking the sleep cycle calculator for bedtime timing or the sleep debt calculator to see how a poor night stacks up over a week.