What is caffeine's half-life?
A substance's half-life is the time it takes for half of it to be eliminated from your body. For caffeine, that's roughly 5 hours in a typical healthy adult, though it ranges from about 3 to 7 hours depending on the person. So if you consume 200 mg, around 100 mg remains after 5 hours, 50 mg after 10 hours, and 25 mg after 15 hours.
How this calculator works
The calculator models caffeine clearance as exponential decay. Starting from your dose, it halves the amount remaining for every half-life that passes, using the formula below.
Remaining caffeine = dose × 0.5 ^ (hours elapsed ÷ half-life). Worked example: 200 mg with a 5-hour half-life, 8 hours later = 200 × 0.5^(8 ÷ 5) ≈ 66 mg still circulating.
Caffeine content of common drinks
To use the calculator, you'll need your caffeine dose. The table below lists typical amounts in popular drinks - actual content varies by brand and serving size.
| Drink | Serving | Caffeine |
|---|---|---|
| Brewed coffee | 240 ml (8 oz) | ~95 mg |
| Espresso | 30 ml (1 shot) | ~63 mg |
| Energy drink | 240 ml (8 oz) | ~80 mg |
| Black tea | 240 ml (8 oz) | ~47 mg |
| Cola | 355 ml (12 oz) | ~35 mg |
| Green tea | 240 ml (8 oz) | ~28 mg |
Caffeine and sleep: when to stop drinking coffee
Because caffeine lingers for hours, an afternoon coffee can still be active at bedtime. Research has found that consuming caffeine even 6 hours before bed can measurably reduce total sleep time. A common rule of thumb is to stop caffeine at least 8–10 hours before you plan to sleep - for an 11 pm bedtime, that means cutting off around 1–3 pm. Use the calculator to see how much will still be in your system at lights-out.
Your caffeine cutoff time, by bedtime
The table assumes a five-hour half-life and aims to leave you under about 50 mg at lights-out, which is roughly the point where most people stop seeing measurable effects on sleep depth. One brewed coffee is about 95 mg; a large shop coffee or a strong pre-workout is closer to 200 mg.
If you are a slow metaboliser, pregnant, taking oral contraceptives, or on certain medications, clearance can take up to twice as long. Push each cutoff earlier by two to three hours.
| Bedtime | Last 95 mg coffee | Last 200 mg dose |
|---|---|---|
| 22:00 | 16:30 | 12:00 |
| 22:30 | 17:00 | 12:30 |
| 23:00 | 17:30 | 13:00 |
| 23:30 | 18:00 | 13:30 |
| 00:00 | 18:30 | 14:00 |
What affects how fast you clear caffeine?
Caffeine metabolism varies a lot between people. Genetics (variations in the CYP1A2 liver enzyme), pregnancy, liver health, certain medications and oral contraceptives can all lengthen caffeine's half-life, while smoking shortens it. If caffeine keeps you awake even when consumed early, you may be a 'slow metaboliser' and should cut off earlier in the day.
Safe daily caffeine limits
For most healthy adults, up to 400 mg of caffeine per day - roughly four cups of brewed coffee - is considered safe by health authorities such as the US FDA. Going much higher can cause jitteriness, a racing heart, anxiety and disrupted sleep. Pregnant people are usually advised to stay under 200 mg per day, and individual tolerance varies widely.
Remember that caffeine adds up across coffee, tea, energy drinks, cola, pre-workout supplements and even chocolate. If you're sensitive, get your caffeine in early in the day so the dose has time to clear before bed - which is exactly what this calculator helps you plan.