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Radioactive Decay calculator

Calculate how much of a radioactive isotope remains after a period of time, using a custom half-life or a common isotope.

How it works Report a problem

Radioactive Decay calculator

Enter your values below and press Calculate to see your result instantly.

Remaining Nuclei (Nₜ)
297.8
DECAY CONSTANT (λ)
0.00012
MASS LOSS
70.2%
After 10,000 units of time, only 29.7% of original matter remains active.

About the Radioactive Decay calculator

Calculate how much of a radioactive isotope remains after a period of time, using a custom half-life or a common isotope.

How to use the Radioactive Decay calculator

  1. Enter the Initial Quantity (N₀) and choose a Predefined Isotope or enter a custom Half-Life (t½).
  2. Enter the Time Elapsed (t).
  3. Press Calculate.

Formula and method

N(t) = N₀ × (½)^(t ÷ t½). Half-lives: carbon-14 about 5,730 years, uranium-238 about 4.47 billion years, caesium-137 about 30 years and iodine-131 about 8 days.

Worked example

A sample that has 25% of its original carbon-14 is two half-lives old: about 11,460 years.

Tips

  • Use the same time unit as the half-life.
  • Carbon dating works for organic material up to roughly 50,000 years old.

Frequently asked questions

Why is carbon-14 used for dating?
Living things absorb it, and after death it decays at a known rate, which lets scientists work out the age.