How to use the half-life calculator
- Choose what to find: the amount left, the starting amount, the time that has passed or the half-life.
- Enter the other three values. Amounts can be in any unit (grams, atoms, becquerels, percent) as long as both use the same one, and the time and half-life share the unit you pick.
- Read the answer, the number of half-lives, the decay constant and mean lifetime, and the table of what remains after each half-life.
Half-lives of some isotopes
| Isotope | Half-life |
|---|---|
| Iodine-131 | 8.02 days |
| Cobalt-60 | 5.27 years |
| Radium-226 | 1,600 years |
| Carbon-14 | 5,730 years |
| Uranium-238 | 4.47 billion years |
Not just radioactivity
Anything that decays by the same fraction in each equal time step follows the same law: the concentration of many medicines in the blood, the charge on a discharging capacitor, or the fall in a light signal through a fibre. The half-life is the time for any amount to halve, whatever the amount was at the start.