Half Life Chemistry Calculator

Any unit: grams, moles, atoms, Bq, etc.
In any time unit (must match elapsed time unit).
Must be in the same units as half-life.
Chemistry

Calculate the remaining amount of a radioactive substance or the number of half-lives elapsed from initial quantity and half-life.

Formula

N(t) = N₀ × (½)^(t/t½); λ = ln(2) / t½
  • N₀ = initial amount; t½ = half-life; t = elapsed time.
  • After n half-lives: N = N₀ × (½)ⁿ.
  • Decay constant λ = ln(2) / t½ ≈ 0.693 / t½.
  • Any consistent units can be used for time.

100 g, half-life 10 min, after 30 min

Inputs
  • Initial Amount: 100
  • Half-Life: 10
  • Elapsed Time: 30

n = 30/10 = 3 half-lives. N = 100 × (½)³ = 12.5 g remaining.

Frequently asked questions

What is half-life?
The time required for half of a radioactive substance to decay.
Does the initial amount matter?
No — after the same number of half-lives, always 50% remains regardless of starting amount.