Evaporation is change of liquid water into vapor. In hydrological cycle, evaporation is a complex phenomenon whose estimation requires great deal of calculations. In this study, it has been tried to present a simple method for calculating potential evaporation, and to calculate evaporation from water surfaces using statistical methods and climatic factors effective in evaporation. In this method, multi-variable regression and linear equation have been utilized. Statistical data of evaporation from tank has been used as function’s variable and factors of temperature and saturation deficit have been taken as independent variables. Finally, numerical values required by equation of line have been calculated using three-variable (three-unknown) equation; thus, the amount of potential evaporation from water surfaces is calculable if temperature and saturation deficit are known. The results from this method are in good agreement with the measured amounts of evaporation from the tank.