We present an engineering analysis of the rate of evaporation of solvent in the hanging drop method of protein crystal growth. Our results are applied to 18 different drop and well arrangements commonly encountered in the laboratory. We take into account the chemical nature of the salt, the drop size and shape, the drop concentration, the well size, the well concentration, and the temperature. We find that the rate of evaporation increases with temperature, drop size, and with the salt concentration difference between the drop and the well. The evaporation possesses no unique half-life. Once the salt in the drop achieves about 80% of its final concentration, further evaporation suffers from the law of diminishing returns.