We present experimental and theoretical methods to predict maximum and working filter capacities for the capture of superparamagnetic microparticles through high-gradient magnetic separation (HGMS). For this, we employed various combinations of nine different HGMS filter matrices and two types of superparamagnetic microparticles. By calculating the separated particle mass per filter mesh area, we clearly demonstrated the influences of wire diameter and wire mesh spacing on the particle build-up density. Here, we introduce a simple experimental method for estimating average build-up densities in HGMS. Together with known physical parameters of the filter matrix and the background field, such average build-up densities allow good predictions of the operational working filter capacities.