Caffeine Calculator

Caffeine does not simply switch off when you finish your cup. It lingers in the bloodstream for hours, continuing to block the adenosine receptors that regulate sleepiness well after the alertness effect fades. This calculator tracks two things most caffeine tools ignore: the running total against daily safety limits, and how much caffeine is still active in your system at any given hour, including when your head hits the pillow.

What This Calculator Measures

This gives you two numbers, not one: where your logged intake stands against the daily safety reference, and an estimate of how much caffeine is still active in your system at a chosen bedtime. The second number is what actually predicts sleep disruption, not the raw daily total.

The Formula

Each drink or dose you log is added to a running daily total, compared against the reference limits used in clinical guidance. The U.S. FDA puts the general population threshold at 400 mg per day for healthy adults, roughly four standard 8 oz cups of brewed coffee. A separate, lower limit of 200 mg per day applies during pregnancy, since caffeine crosses the placenta and the developing fetus cannot metabolize it¹,².

Body weight is optional. If entered, the calculator can express intake as mg per kilogram of body weight alongside the flat limit, which is how most clinical pharmacokinetics research frames dose exposure. The flat 400 mg figure remains the primary reference regardless.

Caffeine clears the body through first-order pharmacokinetics, a fixed proportion of what remains is eliminated per unit time, not a fixed absolute amount, producing an exponential decay curve rather than a straight-line drop³:

Remaining (mg) = Dose (mg) × 0.5 ^ (hours elapsed ÷ half-life)

The default half-life used is 5 hours, the average across healthy, non-pregnant, non-smoking adults in the pharmacokinetics literature.

How This Calculator Computes Your Result

Each dose you log decays on its own curve from the time it was consumed. The calculator sums these individual decay curves to produce a single estimate of total active caffeine at any given hour, the metric that actually determines sleep impact, not the raw daily total³.

From this combined curve, the calculator derives a “last-safe caffeine” time: the point at which another standard dose (95 mg, one cup of brewed coffee) would still clear enough by your target bedtime to keep estimated remaining levels below about 50 mg of active caffeine at bedtime. That 50 mg line is this calculator's own cautious cut-off, not a clinical limit. In a 2025 trial in 23 young men, 100 mg taken 4 hours before bed, which leaves about 57 mg at bedtime at a 5-hour half-life, did not measurably change sleep, while 400 mg disrupted sleep when taken within 12 hours of bedtime⁶.

Caffeine Sources and Typical Values

Default mg values in the dropdown are drawn from published nutrition databases and product data. They reflect typical servings, actual caffeine varies by brewing method, grind, steep time, brand, and serving size.

Custom entry: if your drink is not listed, enter the mg amount directly. Check the nutrition label or the manufacturer’s website, caffeine content is required on the label for supplements and often available for branded beverages.