Calculation of a dose rate at a new distance

Inverse square law dose rate calculator (dose rate at a new distance)

This calculator allows you to estimate the radiation dose rate at a new distance using the inverse square law, based on a known dose rate measured (or specified) at a reference distance.

It is intended to support quick sense-checking, planning stand-off distances, and communicating how dose rate changes with distance in common radiation protection scenarios.

Looking for the inverse calculation?

If you need to work out the distance required to achieve a target dose rate, use the inverse square law distance calculator.

Calculate distance for a target dose rate

What the calculator on this page does

If the dose rate at a reference distance is known, the dose rate at a second distance can be estimated using:

\( \dot{H}_2 = \dot{H}_1 \left(\frac{r_1}{r_2}\right)^2 \)

Where:

  • \(\dot{H}_1\) is the dose rate at the reference distance \(r_1\)
  • \(\dot{H}_2\) is the dose rate at the new distance \(r_2\)

The calculator allows you to select different units for:

  • reference distance (r1r_1)

  • new distance (r2r_2)

  • dose rate units (for input and output)

Supported distance units

  • millimetres (mm)

  • centimetres (cm)

  • metres (m)

  • inches (in)

  • feet (ft)

  • yards (yd)

Supported dose rate units

  • nanosieverts per hour (nSv/hr)

  • microsieverts per hour (µSv/hr)

  • millisieverts per hour (mSv/hr)

  • sieverts per hour (Sv/hr)

  • microrem per hour (µrem/hr)

  • millirem per hour (mrem/hr)

  • rem per hour (rem/hr)

Appropriate use

This calculator is suitable for:

  • stand-off distance planning and “what-if” comparisons;

  • training and communication of distance effects;

  • quick checks when values appear in different unit systems.

It is not intended to replace formal assessments where shielding, attenuation, scatter, geometry, occupancy, time-at-risk, and instrument response must be considered.

Caution — when the inverse square law may not apply

The inverse square relationship is a good approximation where the radiation field behaves like a point source in free space (or sufficiently far from the source compared with source size). It may not be valid (or may be significantly inaccurate) when:

  • you are very close to an extended source (large area sources, long sources, tanks, waste packages);

  • dose rate is influenced by shielding, self-attenuation, or collimation;

  • scatter from walls, floor, or shielding dominates (complex environments);

  • the detector’s energy/geometry response affects readings.

Use this calculator as a supporting tool, and seek competent advice where results are used to inform safety decisions.

Important note

Distances entered must be greater than zero. The calculator will not accept zero or negative distances.

For advice on applying dose-rate information in your workplace, please contact RP Alba Ltd.