Hershfield method is one of the most widely used for PMP calculation. In this study, at first the equations of adjustment factors of Hershfield method including frequency factor (K_m), average and standard deviation for maximum observed event and sample size, fixed observational time intervals and area reduction curves were extracted. After digitizing curves, equations were fitted on curves then the best equations were determined based on R². Based on the extracted equations, a user friendly multi-platform JAVA application called PMP Calculator was designed. In this study the annual maximum 24 hours precipitation during 22-34 years for 7 synoptic and climatological stations located in Gilan province were evaluated. The results indicated that for the first approach, minimum and maximum values of K_m and PMP₂₄ were found to be varied in the range of 14.91-18.69 and 178 -754 mm in this area. The minimum and maximum values of K_m and PMP₂₄ for the second approach were within the range 1.95-4.77 and 65 -363 mm, respectively. The ratio of PMP₂₄ to maximum of 24 hours precipitation, as an independent criteria of climatic conditions was used for comparing PMP in studied stations. The results indicated that the amount of PMP using the second approach is 36.5-48.2 percent of the first approach.