A probability equation based on the proper assumptions of the particle trajectory and fundamental physics has been developed by analyze beam properties such as beam width and intensity distribution for an in situ particle monitor (ISPM). The radius coordinate which has the same intensity and portion of beam area for detection voltage range were analyzed to calculate particle measurement probability. The particle measurement probability is defined at a ratio of entire beam area to specified beam area which decided by detection voltage range. A probability measurement, given as a function of the detection voltage range, was performed 5 times using 200, 300, 500, 700 nm polystyrene latex standard particles at a pressure of 100 Torr with an in-house ISPM. The theoretical calculation results show good agreement with the experimental results and the maximum error is 20% by calculating probability differences between theoretical and experimental values. A calibration method based on the proposed probability equation enables to developed and increase accuracy of ISPM.