Correct estimation of water saturation for hydrocarbon reservoirs is very important. This study is conducted to evaluate the effect of different variables in water saturation calculation. To achieve this aim, fourteen common water saturation equations were selected. Then the factors of formulas and their numerical ranges were gained. At the next stage, the quantity of one percent change of the factors calculated. With the quantity of the variable, the column of variable is formed. Then with using the column of each variable, water saturation are calculated. Afterwards, the consecutive values of Sw are subtracted from each other and positive values of variation are gained. Then the chart of variation in Sw was plotted against each parameter and its variations. Also the chart of Sw was plotted for understanding the relation between different factors and water saturation. At the end, the average of errors in all of the equations was calculated and compared. The comparison showed that porosity among the factors and cementation exponent among coefficient of Archie can cause very meaningful error for water saturation calculation.