Radioactive Decay Activity Calculator
Remaining activity, decay constant and elapsed half-lives.
Enter your values
ⓘ Live results
Results
- 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
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.
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.
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.