Radioactive Decay Activity Calculator
Remaining activity, decay constant and elapsed half-lives.
Enter your values
Enter the starting activity in units.
Enter the half-life in days.
Enter the time elapsed in days.
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 activity
- 177.54227
- Percentage remaining
- 17.7542%
- Half-lives elapsed
- 2.4938
- Decay constant λ
- 0.086427 per unit time
- Mean lifetime
- 11.5704
- Amount decayed
- 822.45773
- Time to reach 1% of start
- 53.284
Quick answer
Activity decays exponentially: A = A₀ × (1/2)^(t ÷ t½), with a decay constant λ = ln2 ÷ half-life.
A = A₀·e^(−λt), λ = ln2 ÷ t½
At a glance
| What it does | Remaining activity, decay constant and elapsed half-lives. |
|---|---|
| Category | Science |
| Inputs needed | Starting activity, Half-life, Time elapsed |
| Main output | Remaining activity |
| Formula | A = A₀·e^(−λt), λ = ln2 ÷ t½ |
| Cost | Free — no sign-up, no download |
How to use the Radioactive Decay Activity Calculator
- 1Enter the starting activity in any unit — becquerels, curies or counts.
- 2Enter the isotope's half-life and use the same time unit for the elapsed time.
- 3Read the remaining activity and how many half-lives have passed.
Inputs explained
- Starting activity(units)
- Enter the starting activity in units.
- Half-life(days)
- Enter the half-life in days.
- Time elapsed(days)
- Enter the time elapsed in days.
Worked example
Using the values the calculator loads with:
Inputs
- Starting activity1000 units
- Half-life8.02 days
- Time elapsed20 days
Results
- Remaining activity177.54227
- Percentage remaining17.7542%
- Half-lives elapsed2.4938
- Decay constant λ0.086427 per unit time
- Mean lifetime11.5704
- Amount decayed822.45773
- Time to reach 1% of start53.284
Frequently asked questions
What is the mean lifetime?
The average time an individual nucleus survives, 1 ÷ λ, which is about 1.44 half-lives.
When is a source considered spent?
A common rule of thumb is ten half-lives, leaving under 0.1% of the original activity.
What do I need to enter into the Radioactive Decay Activity Calculator?
Just 3 values: starting activity, half-life, time elapsed. 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 Activity Calculator work out the answer?
Activity decays exponentially: A = A₀ × (1/2)^(t ÷ t½), with a decay constant λ = ln2 ÷ half-life. It applies the formula A = A₀·e^(−λt), λ = ln2 ÷ t½ and shows the working so you can check each step by hand.
Is the Radioactive Decay Activity 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.