In the calculation of heat exchangers with individual finned tubes or continuous fin heat exchangers, i.e., plate-fin and tube heat exchangers (PFTHEs), it is necessary to determine the efficiency of complex fins. Usually, these are circular or rectangular fins mounted on circular, elliptical, or oval tubes as well as flattened tubes. In case of PFTHEs, the continuous fin is divided into virtual fins, which are rectangular for an inline pipe layout or hexagonal for a staggered pipe arrangement. This paper presents a procedure based on the finite element method for determining the efficiency of fins of any shape placed on tubes of any shape. The article shows examples of calculating the efficiency of virtual fins in most commonly used PFTHEs. The research also assesses the accuracy of determining the efficiency of complex-shaped fins using approximated methods such as the equivalent circular fin method (Schmidt’s method) and the sector method. The efficiency of a fin as a function of heat transfer coefficient for four different, most common geometries in PFTHEs was depicted. An example of determining the efficiency of a hexagonal fin using various methods for air velocities measured at the exchanger inlet was presented.