Accurate sizing of individual nanoparticles is crucial for the understanding of their physical and chemical properties and for their use in nanoscale devices. Optical sizing methods are non-invasive, rapid, and versatile. However, the low optical response of weakly absorbing subwavelength dielectric nanoparticles poses a fundamental challenge for their optical metrology. We demonstrate scalable optical sizing of such nanoparticles based on confocal scanning microscopy. The method is absolutely calibrated by correlating the optical signatures in the scattered pump laser signal to the ground truth nanoparticle sizes measured by an atomic force microscope. Using an air objective with a numerical aperture of 0.9, we measured the sizes of nanodiamond particles ranging from 35 to 175 nm, with an average error of ± 12.7 nm compared to the ground truth sizes. This technique paves the way for the metrology of a wide range of weakly scattering nano-objects for applications in biomedicine, catalysis, nanotechnology, and quantum optics.

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