Half-Life Calculator
Find the remaining amount after decay, or the half-life itself.
Half-life in plain words
The half-life of a substance is the time it takes for half of it to decay or disappear. After one half-life 50% is left, after two 25%, after three 12.5%, and so on.
The formula
N = N₀ × (1/2)t ÷ half-life. If you know the amounts but not the half-life, rearrange it to half-life = t × ln 2 ÷ ln(N₀ ÷ N).
Example
Start with 100 g of a substance with a half-life of 10 days. After 30 days (three half-lives) 12.5 g remains. Going the other way, if 100 g falls to 25 g in 20 days, the half-life is 10 days.
The same maths applies to radioactive decay and to medicines clearing from the body. The time unit you use for the half-life must match the time passed.
Frequently asked questions
Does it work for growth, not just decay?
Not directly. For doubling time, use the compound interest or exponent calculators.
Can the time unit be anything?
Yes, as long as the half-life and the time passed use the same unit.
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