Dynamic light scattering (DLS) is a technique that is widely used to measure dispersed particle size (specifically the hydrodynamic diameter) via direct measurement of the translational diffusion coefficient within the continuous phase. By utilizing the superior accuracy and resolution of multi-angle dynamic light scattering (MADLS®), it is now possible to determine the absolute particle concentration distribution using the Zetasizer Ultra. This provides the user with a fast assessment of the concentration of each population of particles present within their dispersion. Particle concentration measurements are achieved by recording the time averaged photon count-rate scattered by the sample to perform a transformation of the size distribution to derive the absolute number of particles present. In this technical note we describe the method and calculations used to determine particle concentration.