Half-Life Calculator
Calculate how much of a substance remains after a given time, based on its half-life.
Use the same time unit (e.g. years, hours, days) for half-life and elapsed time.
How to use the half-life calculator
- Enter the starting amount of the substance.
- Enter the half-life (the time it takes for the amount to halve).
- Enter how much time has passed — in the same time unit as the half-life.
- The remaining amount updates automatically.
What is half-life?
Half-life is the time it takes for a quantity of a substance (e.g. a radioactive isotope or a drug in the body) to reduce by half. The decay follows an exponential curve: N = N₀ × (1/2)^(t / T), where N₀ is the starting amount, t is elapsed time, and T is the half-life. The amount decreases quickly at first and more slowly over time, but never quite reaches zero.
Frequently asked questions
Does the amount ever reach exactly zero?
Mathematically, no — the amount halves with every half-life that passes, but never reaches exactly zero. In practice, a substance is considered "gone" after a certain number of half-lives (e.g. 10, at which point less than 0.1% remains).
Why must the half-life and elapsed time use the same unit?
Because the formula uses the ratio between them (t / T) — if one is in years and the other in days, the result will be wrong unless you convert to the same unit first.
What's a practical example of half-life?
Carbon-14, used for dating organic material, has a half-life of about 5,730 years. Some drugs have a half-life in the body of just a few hours.