The calculation of diffuse horizonthal illuminance D V outdoors is generalized and interrelated to parameters that characterize the sky luminance distribution function. The derived analytical formula for D V , rel which is D V normalized using luminance in the zenith can be commonly applicable to all 15 ISO-standardized sky types. This makes the modelling of all 15 skies more uniform than ever before. The presented analytical solution is simplified enough because the set of 60 parameters B 1 ,C 1 ,D 1 ,E 1 –B 15 ,C 15 ,D 15 ,E 15 scaling the diffuse illuminance is no more necessary in the new solution. In addition, the analytical formulae substituting the empirical ones are valid for all solar altitudes γ S (including the situations with sun in zenith). On the contrary, the empirical formulae may become inapplicable for γ S >75° and fail when γ S approaches 90°. Compared with detail numerical integration the analytical formula guarantees more rapid calculation of D V , rel .