The present technical note describes a computer tool for determining the horizon brightness diffuse irradiance on a tilted plane in the model of Perez. The applied procedure considers the configuration factor concept, which is calculated using the unit-sphere method. The results obtained with this procedure, which is more exact than the one followed by Perez, are compared with the ones obtained with Perez's proposal. It may be observed that for small angles of inclination differences are up to 36%, and that said differences decrease when the angle of inclination increases. For angles higher than 77.15 degrees, differences become lower than 1%. It may be concluded that the described procedure, together with the computer tool for implementing it, is adequate for determining the horizon brightness irradiance on a tilted plane, once the radiance of this area of the sky dome is known. (c) 2007 Elsevier Ltd. All rights reserved.