For those needing portability, Rad Pro for Desktop works with Windows 8.1/10 tablets. Will not work with Surface tablets running Windows RT.
Calculate? Select Distance Units Select Exposure Rate Units    mR/hrFeet          FeetmR/hrCalculated Final Dose-RateEnter Starting Dose-RateEnter Ending DistanceEnter Starting Distance Inverse Square Law Calculations

Inverse Square Law Formula
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This calculator was first developed for students to demonstrate the inverse square law principal from radiation physics.  As it turns out, it does get quite a bit of use in the field.  The ALARA calculator on this site is more comprehensive, but sometimes this calculator is all that is required for a simple distance to dose-rate relationship demonstration.