When studying compact heat exchangers, determining the heat transfer coefficient from black box measurements is complicated by the need to determine the fin efficiency. Most experimental studies use the fin efficiency correlation of Schmidt to separate the fin efficiency and the heat transfer coefficient. On the other hand, in numerical studies it is possible to determine the fin efficiency and heat transfer coefficient directly. It is known that the Schmidt correlation performs quite well for continuous fin designs. In this study, the applicability of the Schmidt correlation to complex interrupted fin designs such as the louvered fin was investigated. It is shown that the Schmidt correlation results in an overestimation of the fin efficiency. Furthermore, the fin efficiency is often estimated numerically as the ratio of the actual heat transfer rate to the heat transfer rate for an isothermal fin. An explanation as to why this common method is not correct is provided and a new method is proposed. (C) 2013 Elsevier Ltd. All rights reserved.