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Half-Life Calculator

Calculate radioactive decay: the quantity remaining after some time, the time taken, or the half-life itself.

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Half-Life Calculator

Radioactive decay — find remaining quantity, elapsed time, or half-life t½

N(t) = N₀ × (½)t / t½
Please fill in all required fields.

Enter values and hit Calculate
to see the decay curve & results.

Remaining Quantity
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Decay Constant (λ)
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Mean Lifetime (τ)
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Amount Decayed
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% Remaining
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Step-by-Step
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Half-life is decay's clock: the time in which half of a radioactive sample transforms, then half of what remains, on and on. This calculator finds the amount left after any elapsed time, or works backwards to a half-life or an age.

How to use this calculator

  • Enter the starting amount and the substance's half-life.
  • Enter the time elapsed.
  • Press calculate to see the amount remaining.

Why half-life dates the world

Carbon-14's steady decay clocks archaeological finds; uranium isotopes clock the Earth itself. The maths here never reaches zero, the amount only halves toward it, which is why tiny remnants of very old material remain measurable and datable at all.

Frequently asked questions

What is the half-life formula?
Amount left = starting amount × (½)^(time / half-life). After each half-life, the amount multiplies by one half.
After two half-lives, is the sample gone?
No, a quarter remains. After three, an eighth. It shrinks by halves and never quite reaches zero.
Do all isotopes decay at the same rate?
No. Half-lives range from fractions of a second to billions of years, which is why the choice of isotope must match the age being measured.