This paper presents a novel ‘physics-based’ technique for numerically modeling of multi-scale problems, e.g., antennas with metasurfaces operating at mm wavelengths. The metasurface is replaced by an Equivalent Dielectric (ED) medium, whose use in the numerical simulation significantly reduces the computational resources required to model the problem. Furthermore, the proposed method is accurate and does not need sophisticated new code development but can be used in the existing commercially available software instead.