In the case of metallic alloys, which present dendritic structure, the mechanical properties of foundry products depend on primary and secondary arm spacing. For the prediction of microporosities it is necessary to characterize precisely the dendritic structure formed during solidification, to calculate the permeability and also to estimate the radius of the gas bubble to determine the pressure due to gas/metal surface tension. Therefore, it is important in a computational simulation of the solidification processes to use reliable equations to correlate the calculated thermal parameters with primary and secondary dendrite arm spacing. This study presents a numerical and experimental analysis of some models to predict the primary and secondary arm spacing as a function of thermal parameters. Comparison between the numerical and experimental results for Al 4.5 wt% Cu alloy allowed the selection of adequate equations to predict the dendritic spacing during unidirectional solidification. (C) 2005 Springer Science + Business Media, Inc.40715991609