Rapid characterization of gas-borne nanoparticles is important for monitoring aerosol synthesis as well as environmental particle concentrations. Here, the formation of aggregates by viscous flow sintering of amorphous materials (silica, polymers) and grain boundary diffusion sintering of crystalline ceramics (titania, alumina) or metals (Ni, Fe, Ag etc.) is investigated by multiparticle sintering simulations. A scaling law is discovered between average aggregate projected area and equivalent number of constituent primary particles during sintering. This is a relation essentially independent of time, material properties and sintering mechanisms. So the surface area mean primary particle diameter is determined by (on-line) differential mobility analyzer (DMA) and aerosol particle mass (APM) analyzer measurements and this power law for aggregates. This is in good agreement with the primary particle diameter obtained by nitrogen adsorption.