Mutual coupling in an array of different sized narrow rectangular waveguide slots embedded within an infinite PEC plane is estimated by computing the generalized admittance matrix, connecting the coefficients of the modal expansion of the magnetic field to the ones of the electric field. The generic element of the above matrix requires the numerical evaluation of a quadruple integral. In this paper a singularity cancellation approach is used to remove the singularity of the Green’s function and the order of integration is reduced by a suitable changes of variables. In this way self and mutual admittances can be estimated by computing double integrals. A further approximation is introduced in order to compute the latter ones by single integrals saving computational time and resources. The results of such approach are then compared with the ones provided by a commercial numerical simulation tool showing good agreement.