How to use the Half-Life Calculator
The half-life calculator shows how much of a decaying substance remains after a given time.
Open the Half-Life Calculator →
What you need to enter
- Initial quantity
- Elapsed time
- Half-life (same unit)
The formula
Remaining = initial × (1 ÷ 2)^(time ÷ half-life). Decay constant λ = ln 2 ÷ half-life.
Worked example
Using the default values in the calculator:
- Initial quantity: 100
- Elapsed time: 10
- Half-life (same unit): 5
| Remaining quantity | 25 |
| Fraction remaining | 25% |
| Half-lives elapsed | 2 |
| Decay constant λ | 0.13862944 |
| Mean lifetime τ | 7.213475 |
Tips for better results
- Use the same unit for time and half-life.
- Works for radioactive decay and some drugs.
Common mistakes to avoid
- Mixing units.
- Using it for non-exponential processes.