The half-life of a radioactive isotope is the time it takes for half of its atoms to decay. After one half-life 50% remains, after two 25%, after three 12.5%, and so on. This half-life calculator gives the amount left after any time.
The same maths applies to any first-order reaction in chemistry and to how drugs are cleared from the body. Use the same time unit (seconds, days or years) for both the half-life and the time elapsed.
| Starting amount (N₀) | 100 |
| Half-life (t½) | 5730 |
| Time elapsed (t) | 11460 |
| Amount remaining (N) | 25 |
| Percent remaining | 25% |
| Amount decayed | 75 |
| Number of half-lives | 2 |
| Decay constant λ (per time unit) | 0.000120968 |
After two half-lives, 25 g of 100 g of carbon-14 remains.
Try this example →| Starting amount (N₀) | 100 |
| Half-life (t½) | 8 |
| Time elapsed (t) | 30 |
| Amount remaining (N) | 7.43254 |
| Percent remaining | 7.43% |
| Amount decayed | 92.5675 |
| Number of half-lives | 3.75 |
| Decay constant λ (per time unit) | 0.0866434 |
Only 7.43% of the iodine-131 is left after 30 days.
Try this example →The time taken for half of the radioactive nuclei in a sample, or half of a first-order reactant, to decay.
N = N₀(½)^(t/t½). The decay constant is λ = 0.693/t½.
One-eighth, or 12.5%.
No. Radioactive half-life is unaffected by temperature, pressure or the size of the sample.