Expressions are presented for the computation of mutual admittance between rectangular apertures of different size, in a finite planar array. The equations allow any orientation for any of the apertures, with any number of transverse electric (TE) and transverse magnetic (TM) waveguide modes. Furthermore, the expressions also include the effect of dielectric covering of these apertures. A versatile software package, useful for the design and analysis of finite phased arrays, is produced incorporating these features, and its extensive validation is provided.