A nanoparticle size classifier comprising a variable flow rate system with a single diffusion element 3 placed inside a cell 2 containing inlet 1 and outlets 6 and 7, an aerosol filter 8, flow meter 9, pump 10 and pump controller 11, which enables a plurality of measurements to be obtained by means of passing an aerosol through the diffusion element 3 at various flow rates. Preferably, outlet of the diffusion cell is connected to a particle counter 5 via a three-way valve 6. The diffusion element has a net or screen that permit air through but capture some particles. In a preferred embodiment, the nanoparticle size classifier is connected to a PC 12 (a notebook or a palm-size computer) to acquire and process data.