An accurate and efficient analytical method for computing the three-dimensional local void fraction is proposed in the context of discrete particle modeling. it is developed for the general case of unstructured meshes whose use is unavoidable to efficiently simulate modern gas-solid fluidized bed reactors characterized by complex geometries. The method relates the three-dimensional void fraction to several geometrical parameters. This allows the exact voidage evaluation for the frequently occurring case of having particles not wholly contained within one grid cell regardless its shape. Failing to accurately account for these common particle configurations in dense gas-particle systems has proven detrimental to the accuracy of their simulations. (c) 2008 Elsevier Ltd. All rights reserved.