This study is purposed to estimate the pressure drop and thermal performance of three-dimensional heat exchanger units by the porous approximation, considering inertia effects. The full three-dimensional (3D) estimation of the thermal performance of the heat exchanger is very difficult because of the great complexity of the louvered-fin shape and lack of numerical resources. The volume grid used for this study was less than 10 million computational units. In order to verify the interpretation results, a compact heat exchanger having 9500 louver fins and 3D header tanks was tested experimentally and numerically. The simulation results for the air-side pressure drop agree well with the experimental result within 10 %, while the thermal effectiveness of the air was within 9.5 % of the experimental results. The present numerical method can be an effective tool to estimate the performance of multi-louver finned heat exchangers by using limited numerical resources.