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Half-Life Calculator

Solve exponential decay problems for any of the four variables.

  • Free
  • No sign-up
  • Nothing is sent or stored

Use the same time unit as “time elapsed”. Carbon-14 is 5,730 years.

Result

Amount remaining
25
Fraction remaining
25%
Half-lives elapsed
2
Decay constant λ = ln 2 ÷ T
0.000120968 per time unit
How it was calculated
  1. N = N₀ × (½)^(t ÷ T)

The calculation happens instantly in your browser. The numbers you enter are not sent to our servers or saved.

How to use the Half-Life Calculator

  1. Pick what to solve for.
  2. Enter the other values — time and half-life in the same unit.
  3. Read the result, the fraction remaining and the decay constant.

What does this tool do?

A half-life is the time it takes for half of a radioactive sample to decay. After two half-lives a quarter remains, after three an eighth, and so on.

Carbon-14 has a half-life of 5,730 years, so a sample with 25% of its original C-14 is about 11,460 years old. The same maths applies to drug elimination and other exponential decay.

Why use it?

  • Solve for any variable.
  • Works with any time unit.
  • Decay constant included.

The formula

N = N₀ × (½)^(t ÷ T)
t = T × log₂(N₀ ÷ N)

Privacy

The calculation happens instantly in your browser. The numbers you enter are not sent to our servers or saved. There's no account to create and nothing to install.

Last reviewed by the M2Toolkit team.

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