Radioactivity calculator (mass to activity)

This calculator estimates the radioactivity (activity) associated with a known mass of a single radionuclide, using evaluated nuclear data.

It is intended to support radiation protection practitioners who need to interpret or convert between mass-based information (e.g. sampling results, waste characterisation, trace contamination, sealed source records) and activity-based quantities used in regulation, transport, monitoring and reporting.

If you already know the activity of a radionuclide and need to estimate the corresponding mass, use the Radioactivity calculator (activity to mass).

What the calculator on this page does

For a selected radionuclide, the calculator uses the radionuclide’s specific activity to convert a mass into an activity:

  • You select the radionuclide (element and mass number; isomer where relevant).

  • You enter the mass of material (with selectable mass units).

  • The calculator outputs the corresponding activity in your chosen activity unit.

For transparency, the calculator also displays the specific activity value used for the selected radionuclide. This allows users to sense-check results and supports data validation against other references.

Supported units

Activity units

The calculator supports common SI and legacy activity units:

  • becquerels (Bq), kilobecquerels (kBq), megabecquerels (MBq), gigabecquerels (GBq), terabecquerels (TBq)

  • picocuries (pCi), nanocuries (nCi), microcuries (µCi), millicuries (mCi), curies (Ci)

Mass units

Mass may be entered in:

  • nanograms (ng), micrograms (µg), milligrams (mg), grams (g), kilograms (kg), metric tonnes (t)

Note: the tonne unit used is the metric tonne (t) = 1,000 kg, consistent with UK practice.

Data sources and assumptions

This calculator is based on established nuclear data:

  • Half-life data: ICRP Publication 107

  • Atomic mass data: AME2020 (Atomic Mass Evaluation 2020)

Specific activity is derived from these values and is displayed to the user.

Appropriate use and limitations

  • This calculator is intended for single-radionuclide calculations.
    If your material contains a mixture of radionuclides, you will need to perform additional calculations (for example, calculate activity per radionuclide and sum activities where appropriate, or apply mixture rules required by your regulatory or transport framework).

  • The result is calculated using idealised relationships and evaluated constants. It does not account for:

    • chemical form, matrix effects, or measurement uncertainty;

    • inhomogeneity, self-absorption, or analytical recovery;

    • decay chains or ingrowth (unless explicitly accounted for in your source data).

Outputs should not be used as the sole basis for regulatory submissions or compliance decisions without appropriate technical review.

For advice on radioactive substances within your organisation or regulatory framework, please contact RP Alba Ltd.