This paper presents a method for evaluating the intercept factor (γ) of a beam-down parabolic trough collector (BDPTC), accounting for collector design, solar cone, and shading on the aperture due to secondary reflectors. A numerical ray-tracing technique was developed and used for the purpose. Validation was performed by comparing the results with rays traced in a two-dimensional drafting program. The effects of changing the rim-angle and angular misalignment associated with the primary and secondary concentrators for various designs are presented and discussed in detail. The optimum values of the design parameters for low, medium, and high concentration ratio BDPTCs are reported. An online tool for quickly estimating γ is also launched in this paper.