Motivated by x-ray scattering experiments on heteroepitaxially grown thin films, we present model calculations of the diffuse x-ray scattering arising from misfit dislocations. The model is based on the elastic displacements from dislocations whose positions are spatially uncorrelated. These numerical results give support to a phenomenological model [Phys. Rev. B 51, 5506 (1995)] that predicts the scaling of diffuse scattering intensity with perpendicular wavevector, Qz. At low Qz the diffuse width scales inversely with the defect size, which is given by the film thickness due to the effect of the elastic image field, whereas at high Qz the diffuse width is mosaic-like, scaling with Qz. New experimental results for InxGa1−xAs/GaAs are also presented and compared to the model. The calculations are in good agreement with these experiments, as well as other measurements in the literature for high and low dislocation density.