Radioactive decay calculator (date/time to reach target activity)

This calculator determines the date and time at which a single radionuclide will decay to a specified target activity, based on a known initial activity at a defined reference date and time.

It is intended for operational planning where it is helpful to translate decay calculations into a real-world timeline (for example, “when will this source be below 40 kBq?”)

What this calculator does

Given:

  • an initial activity for a single radionuclide;

  • a reference date and time at which that initial activity applies; and

  • a target activity level,

the calculator determines:

  • the date and time at which radioactive decay would reduce the activity to the specified target level;

  • the fraction of the original activity that would be remaining at that point; and

  • the number of half-lives that need to have elapsed.

This is particularly useful when planning activities against calendar schedules, permit limits, or operational constraints.

Typical use cases

This calculator is suitable for:

  • nuclear medicine teams planning administration, storage, or disposal based on activity at a future time;

  • radioactive waste management and decay-in-storage planning, including scheduling onward transfer or disposal;

  • transport and logistics planning where the activity at dispatch time is important;

  • regulatory compliance checks where activity must be demonstrated to fall below a specified threshold at a defined date/time.

Supported activity units

The calculator supports 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.

How date and time are handled

Users enter:

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

  • a target activity level.

The calculator then estimates when the activity would reach the target level, based on the radionuclide half-life.

Important note on time interpretation: the calculator treats the entered date and time as local time on your device/browser. Users should ensure they apply the correct time basis for their operational context (e.g., local time vs recorded times in a clinical system).

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 an optional working section so users can understand and sense-check the calculation steps.

Regulatory context

Date/time-based decay estimates are frequently required in regulated settings, including healthcare, transport, waste management, and environmental compliance.

Users should ensure results are interpreted within their correct regulatory and operational context. For advice on applying radioactive decay calculations within your organisation, please contact RP Alba Ltd.