Half-Life Calculator
Quantity remaining after radioactive decay, the number of half-lives elapsed and the decay constant.
Results
Quantity remaining
25.000000
Percentage
25.000000
%
Half-lives elapsed
2.0000
Decay constant
0.0001209681
Mean lifetime
8266.6426
What this tool does
Half-life makes no assumption about units: put the half-life and the elapsed time in the same unit — years, days, seconds — and the result follows. The default is carbon-14 at 5730 years. The mean lifetime is longer than the half-life because the few long-surviving atoms pull the average up.
Formula
N = N₀ × (½)^(t ÷ T)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
n0 | Initial quantity | — |
hl | Half-life | — |
t | Time elapsed | — |
N | Quantity remaining | — |
PC | Percentage | % |
HN | Half-lives elapsed | — |
DC | Decay constant | — |
MT | Mean lifetime | — |
Worked example
- Initial quantity100
- Half-life5730
- Time elapsed11,460
- Quantity remaining25.000000
- Percentage25.000000 %
- Half-lives elapsed2.0000
- Decay constant0.0001209681
- Mean lifetime8266.6426
Limitations
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
- Mixing units is the most common source of error. Convert every input to the units shown next to each field before calculating.