Radioactive Decay Calculator
Remaining activity after any time.
Enter your values
Enter the initial quantity you are working with.
Enter the half-life in years.
Enter the elapsed time in years.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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)
At a glance
| What it does | Remaining activity after any time. |
|---|---|
| Category | Science |
| Inputs needed | Initial quantity, Half-life, Elapsed time |
| Main output | Remaining quantity |
| Formula | N = N₀ × (1/2)^(t ÷ half-life) |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Initial quantity
- Enter the initial quantity you are working with.
- Half-life(years)
- Enter the half-life in years.
- Elapsed time(years)
- Enter the elapsed time in years.
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.
What do I need to enter into the Radioactive Decay Calculator?
Just 3 values: initial quantity, half-life, elapsed time. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Radioactive Decay Calculator work out the answer?
Radioactive material halves every half-life, decaying exponentially. It applies the formula N = N₀ × (1/2)^(t ÷ half-life) and shows the working so you can check each step by hand.
Is the Radioactive Decay Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.