The fin effects on laminar forced convection of air in the interfin passages of plate-fin heat exchangers are investigated. Steady state fully developed flows in rectangular, trapezoidal, and triangular plate-fin channels are considered. With H1 and T conditions at the partition plates, the conjugate conduction–convection fin problem is solved computationally. The fin effects on the convective Nusselt number are shown to scale by a new dimensionless parameter Ω, which accounts for the attendant fin material and size; its limits describe perfectly conducting and nonconducting fins. Ineffective fins and the consequent reduction in the convective heat transfer coefficient are most pronounced in low fin density cores with longer fins in low-conductivity metal (stainless steel). However, with increasing fin density and shorter fins, the convection performance is virtually the same as that with 100% fin efficiency; the same is the case when fins are made of very high conductivity metal (copper). These results provide design insights for optimizing the conjugate fin-conduction and fluid-flow convection performance in plate-fin compact heat exchangers.