A system for determining the calibration factor uncertainty of a radiation sensor. A computing device accepts a mathematical model of the sensor, sample, and other items affecting the calibration factor, wherein each known mathematical model parameter is assigned its normal dimensions or values and each not-well-known parameter is assigned a variable with an upper and lower limit, and a shape parameter that describes the parameter may vary within or about the limits. Random values consistent with the upper and lower limits and shape parameters for each of the variable parameters in the model are then selected, to create a mathematical model of one possible variation of source-detector measurement configuration. The selection and calibration factor computation are then repeated a large number of times and statistical parameters describing the calibration factor and uncertainty are then computed. Another embodiment performs the steps at different energies for spectroscopic detectors.