Radioactive Decay Calculator
Remaining activity after any time.
Enter your values
Results update instantly as you type — no submit needed.
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.
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.