A simple but remarkably precise geometric pore-filling model is proposed and experimentally validated for the adsorption of proteins at their iso-electric point (pI) in nanoporous materials. Three different globular proteins-lysozyme, myoglobin, and bovine serum albumin-are used as model proteins to study protein adsorption on two types of ordered mesoporous materials-silica and carbon-which allows us to study the effects of protein and surface structure on the protein adsorption mechanism. The geometric pore-filling model confirms that proteins are closely packed inside the pore channels of mesoporous materials, leading to an exceptionally large protein loading capacity. A relationship for the amount of adsorbed protein as a function of protein size, nanopore volume, and pore diameter is derived. The pore space gradually fills up to complete packing of the available pore space at the highest protein concentration. The high precision of the geometric pore-filling model demonstrates its utility to predict the protein adsorption capacity of ordered nanoporous materials.