The present paper investigates a simple correlation for the computation of monthly global solar irradiation on tilted south-facing surfaces. Original data used for this study are those of the Solar Radiation European Atlas, computed for tilted planes by Pr. Page's method. The conversion factor, R, from monthly irradiation on a horizontal plane to irradiation on the inclined surface, is a linear function of the monthly clearness index, K,. Practical coefficients of this correlation are function of geographical latitude, tilt angle and month. They are given for three tilt angle values: 30°, 60° and 90°. A simple polynomial interpolation vs tilt angle is also proposed. Simple conversion charts are given. For most of the months, the standard-deviation of relative differences (between computed and reference values) is smaller than 2%, being larger only during the winter period from November to February (2 to 5%, according to tilt angle).