Laminar, fully developed flow through single- and double-trapezoidal (or hexagonal) ducts is modeled using a finite-difference method. A coordinate transformation is employed to map the irregular flow cross-section onto a rectangular computational domain. Both H1 and T thermal boundary conditions are considered as they represent the fundamental limiting conditions in most practical applications. Solutions for velocity and temperature variations are obtained for a wide range of duct aspect ratios and with four different trapezoidal angles. The friction factor and Nusselt number results show a strong dependence on duct geometry (aspect ratio γ and trapezoidal angle θ). The variations of f Re, Nu H1 , and Nu T with duct aspect ratio for each θ-valued duct are presented in the form of polynomials in γ. These equations describe the computed numerical values within ±2% for single-trapezoidal and within ±1.5% for hexagonal ducts and are of much importance to the design of compact heat exchangers.