Calculate how much of a drug remains in your body after a given time using first-order elimination kinetics, with a full elimination schedule.
Share this calculator
Also in Medication
Health — Medication
A drug's half-life is the time it takes for the concentration in the body to fall by half. Most drugs follow first-order elimination kinetics, meaning the same fraction is eliminated per unit time regardless of the starting dose. This calculator uses that principle to show how much of a dose remains at any point in time, and how long until the drug is substantially cleared.
In first-order kinetics, the amount eliminated per unit time is proportional to the amount present. This produces an exponential decay curve — the drug concentrations halve with each half-life elapsed.
After five half-lives, approximately 97% of the drug has been eliminated — this is the general clinical rule of thumb for "full clearance" of a drug.
C(t) = C₀ × 0.5^(t ÷ t½)
Remaining concentration at time t
Time to 97% clearance ≈ 5 × t½
Rule of five half-lives
Half-lives vary enormously between drugs: aspirin ~3–4 hours; ibuprofen ~2 hours; metformin ~17 hours; sertraline 26 hours; amiodarone ~40–55 days. Always use the manufacturer-specified or prescriber-given half-life for accurate calculations.
Renal or hepatic impairment can extend effective half-life significantly, as can drug interactions that affect metabolic pathways.
Frequently asked questions
No — this calculator models a single dose only. With regular dosing, drug levels accumulate until a steady state is reached (typically after 4–5 half-lives). A full pharmacokinetic model is needed for multi-dose scenarios.
No. Drug interactions depend on mechanism, enzyme pathways, and clinical context — not just residual concentration. Always consult a pharmacist or prescriber before making dosing decisions.
Related
These related calculators come from the same leaf category, nearby sibling categories, or the same top-level topic.
Estimate blood alcohol content (BAC) using the Widmark formula based on drinks consumed, body weight, sex, and hours since drinking started.
Calculate smoking pack-years from cigarettes per day and years smoked, with cumulative exposure category.
Compare your peak expiratory flow reading against the predicted normal for your age, sex, and height, with green, yellow, and red zone guidance.
Convert blood glucose values between mmol/L and mg/dL and interpret common glucose ranges with diagnostic threshold context for fasting and post-meal readings.
