Caffeine Half-Life Calculator
See how long caffeine may stay in your system
Enter how much caffeine you consumed and when you drank it to estimate how much caffeine may remain in your body over time.
Estimated caffeine remaining now
~200 mg
An estimate from the half-life model — not a blood measurement.
- Initial caffeine
- 200 mg
- Time consumed
- Just now
- Remaining now
- ~200 mg
- Approx. remaining
- 100%
- Estimated remaining at bedtime
A chart of estimated caffeine remaining. The same values are listed in the table below.
Hour-by-hour estimates
| Time | Estimated remaining |
|---|---|
| +0h | ~200 mg |
| +2h | ~152 mg |
| +4h | ~115 mg |
| +6h | ~87 mg |
| +8h | ~66 mg |
| +10h | ~50 mg |
| +12h | ~38 mg |
| +14h | ~29 mg |
| +16h | ~22 mg |
| +18h | ~16 mg |
This calculator provides an estimate based on caffeine half-life and is for informational purposes only. Individual caffeine metabolism can vary significantly. This is not medical advice and does not determine when you can sleep.
How long caffeine may last
What is caffeine's half-life?
Caffeine half-life is the time it takes for the amount of caffeine in your body to fall by about half. For many adults that is roughly five hours, so 200 mg can become about 100 mg after one half-life. The range is wide: genetics, pregnancy, smoking, liver health, and some medications can all change how quickly caffeine is cleared.
How long does caffeine last?
Caffeine can still be present long after you stop feeling wired. How alert you feel and how much caffeine may remain are not the same thing. A drink can leave you focused for a few hours while a meaningful amount is still estimated to be in your system later that evening. This calculator estimates remaining caffeine, not how stimulated you will feel.
How does the caffeine calculator work?
The estimate uses exponential decay: after each half-life, remaining caffeine is cut in half. Between those points it falls on a smooth curve, not a straight line. Enter milligrams and the time you drank. Add a bedtime if you want an estimate for later. Times that cross midnight are handled as the next day.
How long does 200 mg of caffeine last?
Using this calculator’s default 5-hour half-life, 200 mg is estimated at about 152 mg after 2 hours, 100 mg after one half-life, and 50 mg after 10 hours. Change the amount or half-life above to see a different curve — these numbers are a model, not a lab result.
| Time after drinking | Estimated remaining |
|---|---|
| 0 hours | 200 mg |
| 2 hours | ~152 mg |
| 4 hours | ~115 mg |
| 5 hours (one half-life) | ~100 mg |
| 8 hours | ~66 mg |
| 10 hours | ~50 mg |
| 12 hours | ~38 mg |
Why does caffeine affect people differently?
Two people can drink the same energy drink and feel it very differently. Age, genetics, pregnancy, smoking, and some medications change how quickly caffeine is metabolized. The calculator gives one estimate from a half-life you can adjust. It cannot tell you what is right for your body.
Can I use this for energy drinks?
Yes. Choose a drink from the dropdown above to use a verified caffeine amount from the Energeia energy drink database, or type the milligrams from the can. You can also open this page with a drink already selected, for example 200 mg.