How to calculate half-life
Remaining = start × (1/2)^(elapsed ÷ half-life)
Worked example. Amount remaining: 6.25.
Example inputs — Starting amount: 100; Half-life: 5; Time unit: Hours; Time elapsed (same unit): 20. Starting values are illustrative.
From George Guju, founder of Calculyn · Educational calculator guidance, not professional advice.
How it works
Each half-life cuts the amount in half. After n half-lives the remaining fraction is (1/2)ⁿ — so decay is fast early and slows as the amount shrinks.
Assumptions & limitations
- Results are rounded for display.
Formula and assumptions reviewed October 2026 by the Calculyn team. How we build our calculators
Frequently asked questions
When is it effectively gone?
After about 7 half-lives less than 1% remains; after 10, less than 0.1%.
Is this used for medications?
Yes — drug dosing uses half-life, but never adjust medication based on a calculator; ask your doctor or pharmacist.