Background and purpose: Sample size and its determination is one of the most important problems in health researches. Calculating sample size for prevalence studies is one of the common questions of sample size topics. Minimum sample size with least complexity is desirable in order to achieve the basic goal of these studies. This study aims to compare two formulas of sample size calculation for prevalence researches and finally, to use the simplest formula to get the most appropriate sample size. Methods Sample size for proportions: 0.9, 0.95, 0.99, 0.999 candidates of p close to 1 proportions 10-5, 10-4, 10-3, 10-2, 0.05, 0.1 candidates of p close to 0, and proportions 0.3, 0.4, 0.5, 0.6, 0.7 candidates of p close to 0.5 were calculated. For comparing n1, n2 φ =n_1⁄n_2 , it was computed by R package (2.10.1). Results Computed sample size by (f2) is lightly greater than sample size computed by (f1) and maximum value of φ index for comparing the two formulas equals 1. Conclusion Results show that the calculated sample size by (f1) is similar to what was obtained by (f2), though, according to its interpretation and easy computation ,it is suggested for all values of p.