Maximum stay time estimator

This calculator estimates the maximum time a person can remain in an area, based on a known external effective dose rate and a chosen target dose. It is intended to support practical radiation protection decision-making, including work planning and time-at-risk control in higher dose rate environments (for example, industrial radiography/NDT work areas). Remember that radiation doses must be kept to levels that are as low as reasonably practicable (ALARP) in order to comply with the UK’s Ionising Radiations Regulations.

The calculator normalises time internally to hours, applies consistent unit conversions, and provides an optional friendly time breakdown (e.g., hours and minutes) to support clear on-the-job communication.

Looking for the inverse calculation?

If you want to estimate the total dose received over a specified exposure duration, use the external radiation dose estimator.

External radiation dose estimator

What the calculator on this page does

Given:

  • an external effective dose rate (\(\dot{E}\)), and
  • a target total effective dose (\(E\)),

the calculator estimates the maximum allowable exposure time (\(t\)) using:

\(t = \frac{E}{\dot{E}}\)

An optional occupancy factor may be applied where exposure is intermittent or where time-at-risk is a defined fraction of a period.

Supported dose rate units

The calculator supports common effective dose rate units used in radiation protection, including SI units and legacy 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)

Conversions use the standard relationship:

  • 1 Sv = 100 rem

Supported target dose units

Target dose can be entered in scalable units to suit different applications:

  • nanosieverts (nSv), microsieverts (µSv), millisieverts (mSv), sieverts (Sv)

  • microrem (µrem), millirem (mrem), rem (rem)

This allows users to work in units consistent with local reporting conventions, client requirements, or legacy documentation.

Output time units and friendly breakdown

The calculated maximum stay time can be displayed in a selectable unit, including:

  • seconds (s)

  • minutes (min) (default)

  • hours (hr)

  • days (d)

  • years (yr), based on 1 year = 365.24 days

A “friendly breakdown” is provided to support operational use (for example, displaying a result as both minutes and seconds, or hours and minutes).

Appropriate use

This calculator is suitable for:

  • planning maximum stay times in areas where dose rate is known or estimated;

  • supporting briefings and task instructions (particularly for NDT/radiography and other short-duration high-dose-rate activities);

  • comparing the effect of different dose constraints or work planning assumptions;

  • training and professional development.

Important note

Results are indicative and assume:

  • the dose rate entered is representative of the exposure period,

  • conditions remain stable (no changes in source strength, shielding, distance, or configuration), and

  • the dose rate is an appropriate measure of external effective dose rate for the scenario.

This calculator does not replace formal dose assessment, personal dosimetry, workplace monitoring, or regulatory submissions. Where required, advice should be sought from a competent Radiation Protection Adviser (RPA) or other suitably qualified professional. Remember that radiation doses must be kept to levels that are as low as reasonably practicable (ALARP) in order to comply with the UK’s Ionising Radiations Regulations.