Lattice Boltzmann numerical simulations were performed to obtain the drag force on 3D fractal objects. The hydrodynamic radius Rh was calculated and compared with the radius of gyration Rg for a series of fractal aggregates to get an insight in to the role of the aggregate size and fractal dimension. We show that R ∼ N1 / Df and Rh / Rg = f (Df). In addition, a new characteristic radius Rβ has emerged from a scaling of numerical results.