Half-Life Calculator

How to use the half-life calculator

1. Choose what to calculate: the amount remaining, the starting amount, the time elapsed or the half-life.
2. Type the other values. Use the same unit of time for the half-life and the time elapsed.
3. The answer appears with the decay constant and the mean lifetime.

Frequently asked questions

What is the formula?

N = N₀ × (1/2)^(t / t½). After one half-life half is left, after two a quarter, and so on.

What is the decay constant?

λ = ln 2 / t½, the fraction that decays per unit of time. The mean lifetime is 1 / λ, about 1.44 half-lives.

How much is left after 10 half-lives?

(1/2)¹⁰, which is about 0.1% of the starting amount.

Can I watch decay happen?

The Radioactive Decay Simulator decays 1,000 atoms at random and plots the count against the exponential curve, for any isotope.