The calculation of water evaporation in pools (e.g., swimming pools) is needed for design and analysis. Numerous empirical equations for calculating water evaporation in undisturbed pools have been proposed and found inaccurate. Shah (1981, 1990, 2002) derived a formula using the analogy between heat and mass transfer that was shown to agree with almost all available data and is widely accepted. However, the complete derivation of this formula was unavailable until now, as only parts were presented in earlier papers. The complete derivation is given here. Results of comparisons with available test data for these and other published formulas for both disturbed and undisturbed pools are also given.Units: SI