Half-Life Calculator

Calculate remaining amount, initial amount, elapsed time or half-life for exponential decay.

Calculator

Use N = N₀ × (1/2)^(t/t½). Time and half-life must use the same unit; the default values are unitless.

Calculate
Remaining amount25
Initial amount100
Elapsed time2
Half-lives elapsed2
Remaining25%

Formula and assumptions

N = N₀ × (1/2)^(t/t½)

This idealized model applies a constant half-life and is intended for learning and calculation checks. It does not assess radiation exposure, isotope handling or safety.

Assumption: The decay rate and half-life are constant over the interval.

Variables and units

  • N — remaining amount
  • N0 — initial amount
  • t — elapsed time
  • t1/2 — half-life

Worked example

After two half-lives, 25% of the initial amount remains.

Important limits

It is a learning model only and must not be used to assess radiation exposure, handling or safety.

Reference and related tools

OpenStax: Nuclear chemistry. Return to Chemistry Calculators for the other tools in this group.