Dairy ultrafiltration permeate is a fluid that is often discarded from the manufacturing process. However, downstream processes such as nanofiltration can be used to separate the valuable lactose and calcium from the monovalent salts, facilitating recovery of a valuable by-product. Previous work has shown that the nanofiltration membrane can suffer from significant fouling due to calcium precipitation. In this work, a model describing the speciation and fouling potential of calcium salts in skim milk ultrafiltration permeate is presented. Results are compared with experimentally determined calcium concentrations in both laboratory prepared solutions and permeate obtained directly from a dairy factory. The model is able to fit the experimental data well for solutions below the saturation concentration for calcium salts, but is less able to fit systems where precipitation of such salts is possible as the extent of the metastable salt region is not readily predicted.