Solution-based synthesis of nanoparticles does not yield monodisperse particles, but rather a well-defined particle size distribution (PSD). There is currently no simple means to anticipate or model these size distributions, which critically affect the properties of the resulting nanomaterials. We simulate the temporal evolution of the PSD in the framework of a nucleation and growth model, with the critical postulate that the coagulation efficiency between two nanoparticles is quantitatively determined by the known, interparticle potential energy. Our simulation based on this ansatz, not only a priori predicts experimentally obtained complete PSDs of uncoated or coated (with poly(acrylic acid)or dextran) iron oxide nanoparticles but also accurately captures the influence of surface coverage of a coating agent on the resulting PSD. © 2016 American Chemical Society.