Radioactive Activity Calculator (Bq & Ci from Mass)
By Santaji GadepatilinUpdated November 2, 2026
Activity is the number of nuclei that decay per second, A = λN, and is measured in becquerels (Bq) or curies (Ci). This radioactive activity calculator works out the decay constant, the number of atoms and the activity of a sample from its mass and half-life, plus how much activity remains later — useful for the sealed Cs-137 and Co-60 sources used in nuclear level measurement.
How to use the radioactive activity calculator
Enter the mass of the radioactive isotope itself (not the whole source capsule), its molar mass, its half-life and, optionally, an elapsed time. The calculator returns the decay constant, the number of atoms and the activity now and after that time, in becquerels and curies. To find the mass of isotope in a source of known activity, enter the activity and clear the mass field.
Activity formula
λ = ln 2 ÷ t½ · N = (m ÷ M) × NA · A = λN
A(t) = A0 × 2−t/t½ · 1 Ci = 3.7 × 10¹⁰ Bq
Example
1 mg of cobalt-60 (M = 59.9338 g/mol, t½ = 5.2714 years):
- λ = 0.693147 ÷ (5.2714 × 31,557,600 s) = 4.1667 × 10⁻⁹ s⁻¹
- N = 0.001 ÷ 59.9338 × 6.022 × 10²³ = 1.0048 × 10¹⁹ atoms
- A = λN = 4.1867 × 10¹⁰ Bq = 1.132 Ci
- After 10 years: A = 4.1867 × 10¹⁰ × 2−10/5.2714 = 1.1241 × 10¹⁰ Bq (0.304 Ci)
Specific activity of common radionuclides
| Isotope | Half-life | Specific activity | Ci per gram |
|---|---|---|---|
| Cobalt-60 | 5.2714 y | 4.187 × 10¹³ Bq/g | 1.13e+03 Ci/g |
| Caesium-137 | 30.08 y | 3.212 × 10¹² Bq/g | 86.8 Ci/g |
| Strontium-90 | 28.79 y | 5.110 × 10¹² Bq/g | 138 Ci/g |
| Americium-241 | 432.6 y | 1.268 × 10¹¹ Bq/g | 3.43 Ci/g |
| Radium-226 | 1,600 y | 3.658 × 10¹⁰ Bq/g | 0.989 Ci/g |
| Carbon-14 | 5,700 y | 1.657 × 10¹¹ Bq/g | 4.48 Ci/g |
| Iodine-131 | 8.025 d | 4.599 × 10¹⁵ Bq/g | 1.24e+05 Ci/g |
| Technetium-99m | 6.007 h | 1.952 × 10¹⁷ Bq/g | 5.27e+06 Ci/g |
| Uranium-238 | 4.468e+09 y | 1.244 × 10⁴ Bq/g | 3.36e-07 Ci/g |
Specific activity here is for the pure isotope; real sources contain carrier material and encapsulation.
Notes
- The half-life field accepts years, days, hours, minutes or seconds.
- Activity alone does not give dose — that also depends on the radiation type, its energy, distance and shielding.
- For half-life arithmetic without atoms or masses, use the half-life calculator.
Frequently asked questions
How do you calculate radioactive activity?
A = λN, with the decay constant λ = ln 2 ÷ t½ (in seconds) and the number of atoms N = (mass ÷ molar mass) × Avogadro's number. The result is in becquerels.
What is the difference between a becquerel and a curie?
1 Bq is one decay per second. 1 Ci = 3.7 × 10¹⁰ Bq, originally the activity of 1 g of radium-226, so 1 mCi = 37 MBq.
How does activity change with time?
It halves every half-life, A(t) = A₀ × 2^(−t/t½). After 10 half-lives about 0.1% of the original activity is left.
What is specific activity?
Activity per unit mass, usually Bq/g or Ci/g. Short half-lives give high specific activity — 1 g of Co-60 is about 1,100 Ci, while 1 g of U-238 is only 12 kBq.