A 1-bit Reconfigurable Intelligent Surface (RIS) element based on PIN diodes is designed in this paper. The physics-based approach is adopted to develop the designed RIS element’s equivalent circuit model (ECM). The ECM is derived from the transmission-line circuit representation of the periodic surface and the microstrip line theory, which incorporates the designed RIS element’s geometry and electrical characteristics. The simplified and practical ECM could calculate the reflection coefficients approximately to those provided by simulations, avoiding repetitive simulation during the early stages of RIS design. Finally, spectral efficiency is evaluated for the designed RIS array used as a reflector for a base station with a directional antenna to serve a rectangular region.