Half-Life Calculator
Work out how much of a substance is left, how long decay takes, or the half-life itself, for radioactive isotopes, medicines in the body and more.
In the same time unit as the time passed, e.g. hours or years.
Amount left
25
- Share left
- 25%
- Half-lives passed
- 2
After 10 time units, 2 half-lives, 25 of 100 is left (25%).
How it's worked out
- N = N₀ × ½^(t ÷ T)
100 × 0.5^(10 ÷ 5)25
The formulas
N = N₀ × ½^(t ÷ T) · t = T × log₂(N₀ ÷ N) · T = t ÷ log₂(N₀ ÷ N)
Worked example
Starting with 100 and a half-life of 5, after 10 time units (two half-lives) 25 is left.
Questions people ask
What is half-life?
The time it takes for half of something to decay or be removed. After two half-lives a quarter is left; after three, an eighth.
How do I calculate the amount left?
N = N₀ × ½^(t ÷ T). 100 units with a 5-hour half-life leave 25 after 10 hours.
How long until a medicine leaves the body?
Roughly 4 to 5 half-lives, when only 3% to 6% is left. Ask your pharmacist about a specific medicine.
About this calculator
Formula and rules last checked 25 October 2026Written and tested by the CalculatorDex teamReport a problem