Caffeine Half-Life Calculator
Enter how much caffeine you consumed and how many hours have passed to estimate how much is still active in your system, based on caffeine's average half-life of about 5 hours.
How the Caffeine Half-Life formula works
Standard exponential decay based on half-life:
Remaining caffeine = Dose × 0.5^(Hours elapsed / Half-life)
Step-by-step calculation
- Divide the hours elapsed by caffeine's half-life (about 5 hours on average).
- Raise 0.5 to that power.
- Multiply by the original dose to find how much remains active.
Worked example
200 mg of caffeine, 5 hours later (one half-life): Remaining = 200 × 0.5^(5/5) = 200 × 0.5 = 100 mg. After 10 hours (two half-lives): 200 × 0.5² = 50 mg.
Frequently asked questions
Does everyone metabolize caffeine at the same rate?
No — caffeine's half-life varies by individual, commonly ranging from about 3 to 7 hours depending on genetics, liver function, pregnancy, certain medications, and other factors. 5 hours is a commonly cited population average, not a universal number.
Why might caffeine affect my sleep even if I drank it hours ago?
Even after one half-life, half the original dose is still active — and it can take several half-lives for caffeine to clear enough to stop affecting sleep for sensitive individuals, which is why caffeine consumed in the afternoon can still disrupt sleep that night.