This note will describe how to perform the most accurate and repeatable MADLS-Particle Concentration measurement on the Zetasizer Ultra. Measuring the particle concentration of a sample is similar to measuring its size, but the sample preparation and the effect of sample measurement properties have a greater impact on the concentration measurement than for a standard Dynamic Light Scattering (DLS) size measurement. We will look at the best methods for preparing your sample, the different parameters that affect the size, and thereby the concentration value.