A critical review on several well-known empirical and theoretical equations that calculate the evaporation rate from large free water surfaces has been carried out. Since up to now there had not been a consensus on which equations were better to employ, a large scattering of evaporation rates has been found. Several theoretical criteria have been undertaken, the scattering has been eliminated and a consensus has been reached. It has also been shown that the consensual equations account for average roughness over the free water surface length and tend to better predict the actual evaporation rate. Pan evaporation may generate considerable differences in relation to a large water surface evaporation, depending on the surface length and the rate of mass transfer.