Many solar energy systems require monthly-average meteorological data, in particular, the total solar radiation, for designing these systems. There are several simplified design procedures for estimating the monthly-average daily total radiation on tilted surfaces facing due south at various orientations. In this paper, the monthly-average radiation on tilted surfaces with different orientations is estimated for Gebze. Calculations are based on measured data on horizontal surfaces between 1988 and 1994 from the TUBITAK-M.R.C, station in Gebze, Turkey.