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Radiation Stay Time Calculator

Calculate maximum allowable work time in a radiation field based on dose rate and dose limit

Free radiation stay time calculator for radiation workers, RSOs, and ALARA coordinators who need to determine how long personnel can remain in a radiation field without exceeding a dose limit. Enter the measured or calculated dose rate at the work position, the applicable dose limit (or an administrative control level), and the calculator returns the maximum stay time in hours and minutes. You can also enter accumulated dose already received during the monitoring period to calculate remaining allowable time. This is the third pillar of ALARA: controlling exposure time. Every pre-job briefing for radiography, source handling, hot cell operations, or contaminated area entry should include a stay time calculation. Under 10 CFR 20.1201, the occupational dose limit is 5 rem (50 mSv) per year for the whole body. Most facilities set administrative control levels well below this, typically 500 mrem per quarter or 100 mrem per task. This calculator lets you work backward from any dose limit to a specific time allowance.

Pro Tip: When multiple workers will rotate through the same job, divide the total task time by the stay time to determine how many workers you need. If the job takes 30 minutes and each worker is limited to 10 minutes, you need at least 3 workers. Build in a 20 percent time margin because real tasks always take longer than planned, especially when workers are rushing to beat a dose clock. Also, calculate stay time from the highest dose rate in the work area, not the average, unless you have a detailed time-motion study for each position.

Calculate the dose rate at a work position using distance

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Reduce dose rate with shielding to extend stay time

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Determine dose rate from a known source activity

Activity to Dose Rate Calculator →

Check if the area requires radiation posting

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Radiation Stay Time Calculator
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How It Works

  1. Enter the Dose Rate

    Enter the measured dose rate at the work position in mR/hr, R/hr, mSv/hr, or μSv/hr. Use the highest reading from the work area survey, not the average, unless you have a detailed time-motion study. If you do not have a survey reading yet, use the Distance Calculator or Activity to Dose Rate Calculator to estimate it.

  2. Set the Dose Limit

    Enter the applicable dose limit for the task. This could be the regulatory annual limit (5,000 mrem/yr), a quarterly administrative limit (500 mrem), a task-specific ALARA goal (100 mrem), or any other control level your facility uses. If the worker has prior accumulated dose, enter that to calculate remaining allowable time.

  3. Review Stay Time

    The calculator shows the maximum time the worker can remain at the stated dose rate without exceeding the dose limit. Results are shown in hours, minutes, and seconds. If stay time is too short to complete the job, consider increasing distance, adding shielding, or using more workers in rotation.

Assumptions

  • Dose rate is assumed constant throughout the stay time (no change in source position, distance, or shielding during the task).
  • Dose is calculated as Rate x Time with no correction for dose rate fluctuations.
  • The dose limit entered is the total allowable dose for the period, reduced by any prior accumulated dose the user enters.
  • Whole-body dose (TEDE) is the default metric. Extremity and lens dose limits are not separately calculated.

Limitations

  • Does not model variable dose rates during a task. Use segment-based calculations for complex time-motion scenarios.
  • Does not track cumulative dose across multiple entries or tasks. The user must enter prior accumulated dose manually.
  • Does not distinguish between deep dose equivalent (DDE) and shallow dose equivalent (SDE).
  • Does not calculate internal dose from airborne contamination or ingestion. This is an external dose tool only.

References

  • 10 CFR 20.1201 - Occupational Dose Limits for Adults
  • 10 CFR 20.1003 - Definitions (including ALARA)
  • NRC Regulatory Guide 8.29 - Instruction Concerning Risks from Occupational Radiation Exposure
  • ANSI N43.3 - General Radiation Safety: Installations Using Non-Medical X-Ray and Sealed Gamma-Ray Sources

Frequently Asked Questions

Under 10 CFR 20.1201, the occupational dose limit for the whole body (total effective dose equivalent, or TEDE) is 5 rem (50 mSv) per year. The limit for any individual organ or tissue is 50 rem (500 mSv) per year. The lens of the eye limit is 15 rem (150 mSv) per year. The skin and extremity limit is 50 rem (500 mSv) per year. Most facilities set administrative control levels at 10 to 20 percent of these regulatory limits to provide margin and demonstrate ALARA.
An administrative dose limit (or action level) is a dose threshold set by the facility's radiation protection program, typically lower than the regulatory limit. Common examples: 500 mrem per quarter, 100 mrem per task, or 2,000 mrem per year. When a worker approaches an administrative limit, it triggers a review by the RSO before additional exposure is authorized. These limits are not regulatory requirements but are part of the facility's ALARA program. Setting them forces planners to optimize jobs before they start rather than relying on the regulatory limit as a backstop.
If the dose rate changes significantly during the task (for example, approaching a source, performing work, then backing away), the most accurate method is to calculate the dose for each segment separately: Dose = Rate x Time for each segment. Sum the segment doses and compare to the limit. For quick estimates, use the highest dose rate for the entire planned task duration. This is conservative and appropriate for pre-job planning. Electronic dosimeters (EPDs) with alarm setpoints provide real-time monitoring during the actual work.
ALARA stands for As Low As Reasonably Achievable. It is a regulatory principle (10 CFR 20.1003) that requires licensees to make every reasonable effort to keep radiation doses below regulatory limits. Time, distance, and shielding are the three practical methods. Reducing stay time directly reduces dose (Dose = Rate x Time). Pre-job stay time calculations are a standard ALARA planning tool. They force the question: can this task be completed within an acceptable dose? If not, the plan needs to change before work begins.
Always use the dose rate at the actual work position where the worker's body (specifically the torso) will be during the task. Contact dose rates on a source container or pipe surface are useful for handling precautions but do not represent whole-body exposure during a task performed at arm's length. Survey the area at the positions where workers will stand, kneel, or reach during the job. If the work position varies, use the highest dose rate in the range of positions.
Electronic personal dosimeters (EPDs) display real-time accumulated dose and dose rate. You can set two alarm thresholds: a dose rate alarm (alerts the worker if they enter an unexpectedly high field) and a cumulative dose alarm (alerts when approaching the task dose limit). The cumulative alarm effectively enforces the stay time limit in real time, even if the dose rate varies during the job. EPDs supplement but do not replace pre-job stay time calculations, which are needed for planning crew size and task sequencing.
Disclaimer: This calculator provides stay time estimates based on the dose rate and dose limit you enter. Actual exposure depends on worker position, dose rate variability, and task duration. Regulatory compliance requires personal dosimetry and calibrated survey instruments. This tool does not replace your facility's radiation protection program or RSO oversight.

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