The work shows how natural illuminance values inside buildings depend on the calculation method of the sky luminance distribution implemented in the computer code which, for a room of fixed geometry and characteristics, transforms the outdoor meteorological data into indoor illuminance data. By means of the INLUX calculation code, experimentally validated, 12 different calculation methods of sky luminance distribution were compared, sometimes obtaining strong differences among the calculated values of indoor illuminance. The choice of some methods instead of others; depends on the meteorological data available in the place considered. Comparing the calculated illuminances with the experimental values measured inside a scale model (1:5), located outdoors in Osaka (Japan), and exposed to daylight, the best calculation methods, among those considered, of sky luminance under overcast sky conditions were identified. (C) 2009 Elsevier Ltd. All rights reserved.