Half-Life Caffeine Calculator
Estimate how much caffeine remains over time. Set your dose or list multiple doses, then choose a half-life. Typical adult half-life is often around 5 hours, but values can differ by person, medication, liver function, pregnancy, and smoking status. This tool is for general information only.
Inputs
Doses
| Label | Dose, mg | Hours ago |
|---|
Formula: remaining = dose × 0.5^(elapsed ÷ half-life). For multiple doses, the calculator sums remaining from each dose at the same evaluation time.
Results
Remaining now
Percent remaining
Time to below threshold
| Time | Remaining, mg | Percent |
|---|
Total consumed
Remaining now
Remaining at bedtime
Time to below threshold
| Hours from now | Remaining, mg | Percent of total |
|---|
This calculator is informational. If you are sensitive to caffeine, pregnant, on medications, or have health concerns, consult a clinician.
Frequently Asked Questions
What is caffeine half-life, and what values should I use?
Half-life is the time for the amount in your body to drop by 50 percent. Typical healthy adults vary from 3 to 7 hours, often about 5 hours. Half-life can be longer in pregnancy or with certain medications, and shorter in smokers. If unsure, start with 5 hours, then adjust if your results consistently feel too high or too low.
How does the calculator figure out what remains after a dose?
It uses exponential decay:
Remaining = Dose × 0.5^(Elapsed ÷ Half-life)
Example: 200 mg, 3 hours elapsed, 5-hour half-life → 200 × 0.5^(3/5) ≈ 132 mg remaining.
How do I handle multiple doses in a day?
Compute each dose separately at the same evaluation time, then add them.
Example: 200 mg taken 4 h ago and 100 mg taken 1 h ago, half-life 5 h →
200 × 0.5^(4/5) + 100 × 0.5^(1/5) ≈ 202 mg total remaining.
How can I find when I will fall below a chosen threshold, for example 50 mg?
Solve for extra time needed after now:
Δt ≈ Half-life × log₂(Dose ÷ Threshold) − Elapsed
Example: Dose 200 mg, elapsed 3 h, half-life 5 h, threshold 50 mg →
Δt = 5 × log₂(200 ÷ 50) − 3 = 7 h. You would be under 50 mg about 7 hours from now.
How should I pick a half-life if I am pregnant, a smoker, or on certain medicines?
Use a longer half-life if pregnant or on medicines that slow caffeine metabolism, and a shorter half-life if you smoke or use medicines that speed metabolism. As a practical start, try 7 to 10 h for late pregnancy and 3 to 4 h for heavy smokers, then adjust to match how you feel.
When should I stop caffeine before bedtime?
With a 5-hour half-life, about 25 percent remains after 10 hours, and about 12.5 percent after 15 hours. Sensitive sleepers often stop 8 to 10 hours before bed. In the calculator, set “hours from now to bedtime” to see what will remain at lights-out.