Radioactive decay calculator (defined elapsed time period)

This calculator estimates the remaining activity of a single radionuclide after a defined period of radioactive decay.

It is intended to support radiation protection practitioners, transport professionals, healthcare users, and environmental duty holders who need to understand how radioactive activity reduces over time for planning, compliance, and sense-checking purposes.

The calculation is based on the internationally accepted radioactive decay law and uses authoritative half-life data for each radionuclide.

What this calculator does

Given:

  • an initial activity for a single radionuclide; and

  • a defined elapsed time period,

the calculator determines:

  • the remaining activity after decay;

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

  • the number of half-lives elapsed during the decay period.

These outputs together provide both a regulatory quantity and an intuitive radiation protection check.

Supported activity units

Users may enter and display activity in commonly used SI and legacy 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 being converted for display.

Defining the elapsed time

The elapsed decay period may be defined using one or more of the following fields:

  • years

  • days

  • hours

  • minutes

  • seconds

This allows users to enter decay periods in a natural and intuitive way, reflecting how decay problems are often considered in practice.

Appropriate use

This calculator is suitable for use when:

  • assessing decay-in-storage times for radioactive waste;

  • planning transport or disposal activities following a known decay period;

  • estimating activity reductions over defined operational or reporting intervals;

  • supporting education, training, and professional development.

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 significant, 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 tools 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, enabling users to verify results and understand how outputs have been derived.

Regulatory context

Understanding radioactive decay over time is relevant to many UK regulatory frameworks, including:

  • radioactive waste management and decay-in-storage;

  • transport of radioactive material (ADR Class 7);

  • environmental permitting and compliance assessments;

  • occupational exposure planning and control.

Users should ensure that results are interpreted within the correct regulatory and operational context.

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