About the Half-Life Calculator
What is the Half-Life Calculator?
The half-life is the time for half of a substance to decay. This calculator works it out from two measurements – the starting amount and the amount left after a known time.
It applies to radioactive decay and to any first-order process, including many drug eliminations and chemical reactions.
How to use the Half-Life Calculator
- Initial Amount (sample: 100)
- Amount Remaining (sample: 25)
- Time Elapsed (sample: 20)
- Time Unit (sample: Days)
- Press Calculate Half-Life (results also refresh as you type). The result card shows the Half-Life, along with Decay constant λ, Mean lifetime and Half-lives elapsed.
Formula used
t½ = t × ln 2 ÷ ln(N₀ ÷ N)
- N₀, N = initial and remaining amounts
- t = time elapsed
Use the same unit for both amounts; the half-life comes out in the time unit you choose.
Worked example
These are the sample values already filled in the calculator above, and the result it returns for them:
| Input | Sample value |
|---|---|
| Initial Amount | 100 |
| Amount Remaining | 25 |
| Time Elapsed | 20 |
| Time Unit | Days |
| Result | Value |
|---|---|
| Half-Life | 10 days |
| Decay constant λ | 0.069315 per day |
| Mean lifetime | 14.427 days |
| Half-lives elapsed | 2 |
Tips for using the Half-Life Calculator
- Measure over at least one half-life for an accurate result.
- Doctors use drug half-lives to plan dosing intervals.