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Radioactive decay calculator
Find how much activity is left after any amount of time. Pick the isotope, enter the starting activity, and set the calibration and use times.
Use a custom half-life instead
Runs in your browser. Nothing you enter is sent or saved.
How it works
Radioactive decay follows the half-life: every half-life, half of the remaining activity is gone.
A = A0 × 2−t / T½
Here A0 is the activity at calibration, t is the time elapsed and T½ is the half-life. Set the end time earlier than the start to back-decay to an earlier time.
Common half-lives
| Isotope | Half-life | Typical use |
|---|---|---|
| F-18 | 109.77 minutes | PET imaging |
| Tc-99m | 6.0067 hours | Nuclear medicine imaging |
| Mo-99 | 65.94 hours | Tc-99m generators |
| I-131 | 8.0252 days | Thyroid therapy |
| Lu-177 | 6.647 days | Radioligand therapy |
| Ir-192 | 73.829 days | HDR brachytherapy, radiography |
| Co-57 | 271.74 days | Dose calibrator checks |
| Cs-137 | 30.08 years | Gauges, calibration sources |
Half-lives are NNDC/ENSDF evaluated values. CurieLedger's calculations are checked automatically against these references.
Questions
How many half-lives until a source is "gone"?
After 7 half-lives less than 1% remains. After 10 half-lives, 1/1024 (about 0.1%) remains, the usual rule of thumb for decay-in-storage. Use the decay-in-storage calculator for disposal dates.
Curies or becquerels?
Both work. 1 mCi = 37 MBq. Use the curie–becquerel converter for any unit, including dpm.