This European Standard specifies a calculation model to estimate the sound insulation or the sound pressure level difference of a façade or other external surface of a building. The calculation is based on the sound reduction index of the different elements from which the façade is constructed and it includes direct and flanking transmission. The calculation gives results which correspond approximately to the results from field measurements according to EN ISO 140-5. Calculations can be carried out for frequency bands or for single number ratings. The calculation results can be used also for calculating the indoor sound pressure level due to for instance road traffic ; this use is treated in the informative annex D. This document describes the principles of the calculation model, lists the relevant quantities and defines its applications and restrictions. It is intended for acoustical experts and provides the framework for the development of application documents and tools for other users in the field of building construction, taking into account local circumstances. The model is based on experience with predictions for dwellings ; it can also be used for other types of buildings provided the dimensions of constructions are not too different from those in dwellings.