The rates of water evaporation from indoor and outdoor swimming pools in active use have been measured and compared with evaporation rates from unoccupied pools and with values calculated by the equation W=(95+0.425 v) (pw-pa)Y, where W is evaporation rate, lb/h ft²; v is air velocity at water surface, ft/min.; pw is saturation vapor pressure at water temperature, in. Hg; pa is saturation vapor pressure at air dewpoint, in. Hg; and Y is latent heat at pool temperature, Btu/lb. In undisturbed pools, evaporation rates were measured and found to be 74% of the rates obtained by use of the equation. Rates of evaporation from pools in active use increase with the number of swimmers, rising 40% to 70% above the rates from a quiet water surface. Measurements of evaporation from a pool in use by 15 to 20 swimmers per 1,000 ft² were found to average 26% higher than the rate calculated by the equation.

Units: I-P