An algorithm has been developed in MATLAB, with user interface, to determine the transmission loss (TL), the STC and the acoustic reduction index R of composite walls of n number of surfaces that can also be single-wall, double-wall or two-layer laminate. To validate the results of the single isolation value, a prediction of a composite wall consisting of a brick wall and a double window with an air chamber, was made in the algorithm, obtaining an STC of 35; in-situ measurement, in accordance with the standard UNE-EN ISO 16283-1: 2014, with an STC of 30; and a simulation by the finite element method (FEM) in 2D, having a result in the STC of 32. According to the validations carried out, the developed algorithm is useful for the determination of the sound insulation in composite walls during the first design phase in architectural acoustics.