Mobile soil colloids play an important role in affecting soil hydraulic properties and soil formation, as well as in facilitating the transport of strongly sorbing constituents (e.g., contaminants, pesticides, nutrients) in subsurface environments. Accurate quantification of mobile colloids is thus essential for understanding and prediction of soil element migration, changes in soil hydraulic properties, and colloid-associated contaminants in natural and managed systems. However, quantification of colloids in field samples, especially the <0.45-μm fractions, which are traditionally considered dissolved solutes, has largely been hindered due to the lack of reliable methods. In this study, we developed a simple and efficient methodology of using size-dependent correlations between nephelometric turbidity and mass concentrations of colloids for quantifying colloids in the <0.1-, 0.1- to 0.45-, and 0.45- to 1.0-μm fractions. The correlations were measured using model colloids (latex, silica, and Fe oxide particles) and soil colloids extracted from 37 soils. We found that colloid size strongly affected concentration–turbidity relationships, while colloid composition played a less important role in shifting the correlations. The size-dependent concentration–turbidity correlations were further tested against gravimetric measurements using additional field samples and found to be more accurate than correlations that do not consider size effects. The relatively insignificant particle composition effect indicates the practically “universal” applicability of the reported correlations. In addition, the correlations, for the first time, allow quantification of colloids in different size fractions in environmental samples. This would enhance our capability to more accurately quantify the colloidal pools in natural systems, which have strong implications for understanding the processes and mechanisms of colloid and colloid-associated-constituent mobilization and transport.