For those needing portability, Rad Pro for Desktop works with Windows 8.1 tablets (just tested). 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.