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Radiation Distance Calculator

Apply the inverse square law to find dose rate at any distance from a point source

Free radiation distance calculator for radiation safety officers, industrial radiographers, and health physicists who need to determine how dose rate changes with distance from a sealed source. Enter a known dose rate at a known distance and the calculator returns the dose rate at any new distance using the inverse square law: I2 = I1 x (D1/D2)². You can also solve in reverse, entering a target dose rate to find the minimum standoff distance required. Results display in mR/hr, R/hr, mSv/hr, and μSv/hr. This is the foundational calculation in every radiation protection program. Maintaining distance is one of the three pillars of ALARA (As Low As Reasonably Achievable), along with time and shielding, as defined in 10 CFR 20. Every pre-job survey for industrial radiography under 10 CFR 34 starts with an inverse square calculation to establish boundary distances. The calculator handles the arithmetic so you can focus on the safety decisions.

Pro Tip: The inverse square law assumes a point source in free air with no scatter. In practice, backscatter from floors, walls, and equipment can add 10 to 30 percent to measured dose rates at close range. When you calculate a boundary distance, add a 25 percent margin before posting your restricted area. If you are working near concrete walls or inside a pipe trench, take a field survey to confirm your calculated values.

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Radiation Distance Calculator
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How It Works

  1. Enter Known Dose Rate and Distance

    Enter the measured or documented dose rate at a known distance from the source. This is typically taken from a survey meter reading or from the source manufacturer's certificate. Select the appropriate units for both dose rate and distance.

  2. Enter the Target Distance

    Enter the distance where you need to know the dose rate. This could be a proposed work position, a boundary line, or an unrestricted area location. The calculator applies the inverse square law to compute the dose rate at this new distance.

  3. Review Results or Solve for Distance

    The output shows the dose rate at your target distance in multiple units. To work in reverse, enter a target dose rate (such as 2 mR/hr for an unrestricted area boundary) and the calculator returns the minimum standoff distance required.

Assumptions

  • The source is treated as a point source (physical dimensions are small compared to the measurement distance).
  • No shielding or attenuation is present between the source and the calculation point.
  • Scatter contributions from nearby surfaces are not included in the calculation.
  • The medium between source and detector is air (no significant air attenuation at typical working distances).

Limitations

  • Does not apply to line sources, area sources, or extended source geometries.
  • Does not account for scatter, buildup, or reflection from nearby surfaces.
  • Does not include shielding attenuation. Use the Radiation Shielding Calculator for shielded scenarios.
  • Does not model energy-dependent detector response or calibration factors.

References

  • 10 CFR 20 - Standards for Protection Against Radiation (U.S. Nuclear Regulatory Commission)
  • 10 CFR 34 - Licenses for Industrial Radiography and Radiation Safety Requirements for Industrial Radiographic Operations
  • ANSI N43.3 - Installations Using Non-Medical X-Ray and Sealed Gamma-Ray Sources, Energies Up to 10 MeV
  • Radiological Health Handbook (U.S. Department of Health, Education, and Welfare, 1970)

Frequently Asked Questions

The inverse square law states that radiation intensity from a point source is inversely proportional to the square of the distance from the source. If you double your distance from a source, the dose rate drops to one quarter. If you triple the distance, the dose rate drops to one ninth. This relationship holds for any source that is small compared to the measurement distance and where there is no shielding or significant scatter between the source and the measurement point.
The law breaks down in three common situations. First, when the source is not a point source, meaning you are close enough that the physical size of the source matters (a line source like an activated pipe follows a 1/r relationship, not 1/r squared). Second, when there is significant scatter from nearby surfaces that adds to the direct beam. Third, when there is intervening shielding that attenuates the beam. In practice, the inverse square law is a good approximation whenever you are at least three times the largest source dimension away from the source.
Under 10 CFR 20.1301, the dose limit for individual members of the public is 100 mrem per year (1 mSv/yr). For practical boundary calculations during industrial radiography under 10 CFR 34, most licensees use 2 mR/hr at the restricted area boundary. This value accounts for short exposure durations and provides margin against the annual limit. Some state regulations specify different boundary dose rates, so check your specific license conditions and the applicable state regulations.
The most reliable method is a direct survey with a calibrated ion chamber or GM detector at a known distance. For sealed sources, the manufacturer's certificate lists the activity at a reference date, which you can convert to a dose rate using the specific gamma ray constant (see the Activity to Dose Rate calculator). If you use the certificate, remember to decay-correct the activity to today's date before calculating the dose rate, especially for shorter-lived isotopes like Ir-192 with a 73.8-day half-life.
Distance reduces dose rate by the square of the increase. Doubling distance cuts dose to 25 percent. A 10-foot extension handle on a source projector gives a 100-fold reduction compared to holding the source at 1 foot. Shielding requires material and weight. Time reduction has practical limits because the work still needs to get done. Distance is free, always available, and mathematically the most efficient way to reduce exposure during source handling and retrieval operations.
Disclaimer: This calculator applies the inverse square law for point sources in free air. Actual dose rates depend on source geometry, scatter conditions, and intervening shielding. Field surveys with calibrated instruments are required for regulatory compliance. This tool does not replace the judgment of a qualified radiation safety officer.

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