Radioactive Decay Calculator
Remaining activity after any time.
Enter your values
ⓘ Live results
Results
- Remaining quantity
- 103.1603
- Fraction remaining
- 20.632%
- Decay constant
- 0.13153 /yr
- Half-lives elapsed
- 2.28
Quick answer
Radioactive material halves every half-life, decaying exponentially.
N = N₀ × (1/2)^(t ÷ half-life)
How to use the Radioactive Decay Calculator
- 1Enter the initial quantity or activity.
- 2Enter the isotope's half-life.
- 3Enter elapsed time in the same units.
Worked example
Using the values the calculator loads with:
Inputs
- Initial quantity500
- Half-life5.27 years
- Elapsed time12 years
Results
- Remaining quantity103.1603
- Fraction remaining20.632%
- Decay constant0.13153 /yr
- Half-lives elapsed2.28
Frequently asked questions
What is a decay constant?
ln(2) divided by the half-life; it sets the instantaneous decay rate.
Does decay ever reach zero?
Mathematically no, but it becomes undetectable after about ten half-lives.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.