Calculation of distance required to achieve a target dose rate

Inverse square law distance calculator (distance required to achieve a target dose rate)

This calculator allows you to estimate the distance at which a specified radiation dose rate would occur, using the inverse square law, based on a known dose rate at a reference distance.

It is intended to support stand-off distance planning, task design, and communication of distance-related dose-rate effects in radiation protection scenarios.

Looking for the inverse calculation?

If you already know two distances and want to estimate the dose rate at a new distance, use the inverse square law dose rate calculator.

Calculate dose rate at a new distance

What the calculator on this page does

If the dose rate at a reference distance is known, the distance required to achieve a lower (or higher) target dose rate can be estimated using:

\(r_2 = r_1 \sqrt{\frac{\dot{H}_1}{\dot{H}_2}}\)

Where:

  • Ḣ₁ is the dose rate at the reference distance r₁
  • Ḣ₂ is the target dose rate
  • r₂ is the distance at which the target dose rate would occur

The calculator allows you to select different units for:

  • reference distance (r₁);
  • target distance output (r₂);
  • dose rate units (for both input and target dose rate).

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:

  • estimating stand-off distances to achieve a specified dose-rate criterion;

  • task and work-planning discussions;

  • training and education on the effect of distance on radiation exposure;

  • quick sense-checking of values expressed in different unit systems.

It is not intended to replace detailed assessments where shielding, attenuation, scatter, occupancy, exposure time, or source geometry must be taken into account.

Caution — when the inverse square law may not apply

The inverse square relationship assumes radiation behaves as if it originates from a point source in free space. It may not be valid (or may be significantly inaccurate) when:

  • the source is extended (large, distributed, or elongated sources);

  • measurements are made very close to the source;

  • shielding, self-attenuation, or collimation significantly influences dose rate;

  • scatter from surrounding structures dominates the radiation field.

Professional judgement should always be applied when using inverse square calculations to inform safety-related decisions.

Important note

Distances and dose-rate values must be greater than zero. The calculator will not accept zero or negative values.

Results are indicative only and should be used as a supporting tool rather than as the sole basis for compliance or safety decisions. Where appropriate, advice should be sought from a competent Radiation Protection Adviser (RPA) or other suitably qualified professional.