The louvered fin geometry is widely used for heat transfer to air in automotive heat exchangers. The only published correlation for heat transfer and friction in the corrugated louver fin geometry is that of Davenport. However, this correlation is strictly empirical, since it is based on a multiple regression correlation using dimensional parameters assumed to be important. Further, the correlation does not include the dimensions of the inlet/exit and the internal flow redirection louvers. The objective of the present work is the development of an improved, rationally based correlation. The correlation is based on dividing the total fin surface into four different regions, and applying rationally based heat transfer and friction equations to each of these regions. The resulting correlation is semi-analytical, since it is based on heat transfer mechanisms and theoretical relationships expected to apply the various regions of the fin. A similar procedure is used for the friction correlation. The heat transfer (and friction) correlations each contain only one empirical constant. The development of the correlation is illustrated using the data on 32 core geometries published by Davenport. The standard deviation of the heat transfer and friction correlations are 7.0% and 9.0%, respectively. In terms of maximum deviations, 97% of the data are predicted within ±20%, with 81% of the data predicted within ±10%. Because the fin geometries tested by Davenport do not contain near-parallel fins, better results are expected using a database of commercially typical louver fin geometries.