Radioactive decay calculator (specified dates)

This calculator estimates the remaining activity of a single radionuclide between two specified dates and times.

It is designed for situations where radioactive material is handled, transported, or received at known times, and where it is important to understand how much decay has occurred between a reference time and a later point in time.

What this calculator does

Given:

  • an initial activity for a single radionuclide at a known reference date and time; and

  • a specified target date and time,

the calculator determines:

  • the remaining activity at the target date and time;

  • the fraction of the original activity remaining, expressed as a percentage; and

  • the number of half-lives elapsed between the two times.

This approach allows users to perform decay calculations that are directly tied to real operational timelines rather than abstract time periods.

Typical use cases

This calculator is particularly useful for:

  • nuclear medicine departments assessing activity levels at the time of receipt or administration;

  • checking compliance with permit or licence limits at a specific point in time;

  • planning handling, storage, or disposal activities for short-lived radionuclides;

  • audit and record-keeping where decay calculations must be traceable to calendar dates.

Supported activity units

The calculator supports both SI and legacy activity units, including:

  • becquerels (Bq)

  • kilobecquerels (kBq)

  • megabecquerels (MBq)

  • gigabecquerels (GBq)

  • terabecquerels (TBq)

  • petabecquerels (PBq)

  • picocuries (pCi)

  • nanocuries (nCi)

  • microcuries (µCi)

  • millicuries (mCi)

  • curies (Ci)

  • kilocuries (kCi)

  • megacuries (MCi)

All calculations are performed internally using SI units before conversion for display.

Defining the decay period

The decay period is defined by selecting:

  • a reference date and time (when the initial activity applies); and

  • a target date and time (when the remaining activity is required).

The calculator automatically determines the elapsed time between these two points and applies the appropriate decay calculation.

Important limitations

This calculator applies to single radionuclides only and assumes simple exponential radioactive decay.

It does not account for:

  • radioactive decay chains;

  • daughter ingrowth;

  • secular or transient equilibrium;

  • chemical or biological behaviour.

Where these effects are relevant, or where regulatory decisions depend on detailed decay modelling, advice should be sought from a suitably qualified professional (such as an RPA or RWA), and validated decay modelling software should be used.

Data sources and transparency

Radionuclide half-life data are taken from ICRP Publication 107.

The calculator displays the half-life used and provides a transparent breakdown of the calculation steps, allowing users to understand and verify how results have been derived.

Regulatory context

Date-based radioactive decay calculations are commonly required in:

  • healthcare and nuclear medicine practice;

  • radioactive waste management and decay-in-storage;

  • transport planning and compliance;

  • environmental permitting and regulatory reporting.

Users should ensure that calculated values are interpreted within their correct regulatory and operational context.

For advice on applying radioactive decay calculations within your organisation, please contact RP Alba Ltd.