An experimental study is presented of evaporative free-surface natural convection. A power law relationship is developed between the Sherwood number for evaporation (Sh) and the Rayleigh number for air-side natural convection (Ra). Evaporation of water was investigated in sixteen different tanks having four depths and four widths. Evaporation rates and the relevant temperatures and relative humidity were measured, from which the air-side Sherwood and Rayleigh numbers were obtained. The resulting power law is Sh similar to Ra-0.321. This power law is compared to those obtained by other researchers. A brief discussion of the possible effect of water-side natural convection on evaporation is also presented. (C) 2009 Elsevier Ltd. All rights reserved.