Drug Half-Life Calculator
Calculate the remaining dose of a medication in your system based on initial dose, half-life, and time elapsed.
Your data stays in your browser. We do not store, collect, or transmit any information you enter.
Remaining Dose
Drug half-life is the time it takes for the concentration of a drug in your body to reduce by half.
Remaining Dose = Initial Dose × (0.5)^(Time Elapsed / Half-Life)
Number of Half-Lives = Time Elapsed / Half-Life
Common medication half-lives:
- Ibuprofen: 2–4 hours
- Acetaminophen: 2–3 hours
- Aspirin: 3–5 hours
- Amoxicillin: 1–1.5 hours
- Caffeine: 3–7 hours
- Melatonin: 0.5–1 hour
- Diazepam (Valium): 20–100 hours
General rules:
- After 5 half-lives, ~97% of the drug is eliminated
- After 7 half-lives, ~99% is eliminated
- Steady state is typically reached after 4–5 half-lives of regular dosing
Note: This is a simplified model. Actual drug metabolism depends on liver function, kidney function, age, weight, interactions, and other individual factors. Always follow your prescribing doctor’s instructions.