Caffeine Calculator — How Much Caffeine Is Left in Your System
Enter how much caffeine you consumed, how many hours ago, and your body weight — get the estimated milligrams still active in your body, how long until it drops below a negligible level, and how your intake compares to the FDA's 400 mg daily guideline.
mg
hrs
kg
After exponential decay with a 5.7-hour average half-life
- 1
Elapsed time ÷ half-life
2 ÷ 5.7 = 0.351 - 2
Decay factor
0.5^0.351 = 0.7841Fraction of the original dose still present after exponential decay. - 3
Caffeine remaining
200 × 0.7841 = 156.8
How does this calculator work?
Remaining caffeine = dose × (0.5)^(hours / 5.7). The population-average half-life is 5.7 hours, so a 200 mg coffee falls to ~100 mg after 5.7 h and ~10 mg after ~19 h. The FDA recommends no more than 400 mg/day for healthy adults. Genetics, medications, and liver health can shift your personal half-life significantly.
Formula
How this is calculated
Caffeine is metabolised mainly in the liver by the enzyme CYP1A2 and follows first-order kinetics — a fixed fraction is eliminated per unit time. The population-average half-life is approximately 5–6 hours; this calculator uses 5.7 hours, a commonly cited midpoint. Applying the exponential-decay formula C(t) = C₀ × (½)^(t / t½) gives the concentration remaining after any elapsed time. To estimate when caffeine falls below 10 mg (a negligible threshold), the formula is inverted: t = t½ × log₂(remaining / 10).
Body weight contextualises the dose via the 3 mg/kg stimulant threshold — the approximate intake above which caffeine effects are typically noticeable in the average adult. Below that threshold, effects are mild; sustained doses above 400 mg/day are where the FDA cautions healthy adults.
Important limitations: the 5.7-hour half-life is a population average. Genetic variation in CYP1A2 means "fast metabolisers" may clear caffeine in 3–4 hours while "slow metabolisers" take 8–10 hours. Pregnancy, oral contraceptives, liver disease, and certain medications significantly slow clearance. Caffeine content of drinks varies widely by brand, roast, and brewing method; the hint values are typical but not exact.
Frequently asked questions
With a 5.7-hour half-life, caffeine falls to half its starting dose after about 5.7 hours. A 200 mg dose drops below 50 mg after roughly 11 hours and below 10 mg after about 19 hours. Full elimination takes approximately 28 hours (five half-lives), though amounts below 10 mg have no perceptible effect.
The FDA and EFSA both cite 400 mg/day as a level not generally associated with dangerous effects for healthy adults — roughly 4 cups of brewed coffee. Pregnant individuals are advised to stay below 200 mg/day; children and adolescents should consume significantly less or none.
Genetic variants in CYP1A2 (metabolism rate) and ADORA2A (adenosine-receptor sensitivity) strongly influence caffeine response. Fast metabolisers with low-sensitivity receptors may feel little effect; slow metabolisers with sensitive receptors can experience anxiety or insomnia from small doses.
TG we-Calculate Editorial Team. (2026). Caffeine Calculator — How Much Caffeine Is Left in Your System [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/caffeine-calculator
TG we-Calculate Editorial Team. "Caffeine Calculator — How Much Caffeine Is Left in Your System." TG we-Calculate. 2026. https://we-calculate.com/calculator/caffeine-calculator.
TG we-Calculate Editorial Team, "Caffeine Calculator — How Much Caffeine Is Left in Your System," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/caffeine-calculator
@misc{wecalculate_caffeine_calculator, title = {Caffeine Calculator — How Much Caffeine Is Left in Your System}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/caffeine-calculator}}, year = {2026}, note = {TG we-Calculate} }
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