It is widely reported in the literature that fluidized nanoparticle agglomerates have a mass fractal dimension or about 2.5. In this paper, we question the current methods reported in the literature to calculate the fractal dimension for fluidized nanoparticle agglomerates, which assume a prefactor one in the fractal scaling law. The fractal dimension 2.5 obtained with a prefactor one approximates the density of the agglomerates in a limited range of sizes but does not describe the scaling of the agglomerate density (or agglomerate mass) with the agglomerate size, which is the ultimate meaning of a mass fractal dimension. By studying the settling of fluidized agglomerates, we have found that the prefactor of the fractal scaling law for large fluidized agglomerates can be up to two orders of magnitude larger than one. We show with a simple equation that this large prefactor comes from the multidimensional nature of fluidized nanoparticle agglomerates, revealing that the assumption of a prefactor one can lead to an erroneous fractal dimension when studying multidimensional structures. The fractal dimension found for agglomerates larger than 40 mu m without imposing a value for the prefactor, this is, from the fitting log(density) vs. log(size) is about two. (C) 2014 Elsevier Ltd. All rights reserved.