To minimize formation damage caused by drill-in and completion fluids, bridging agents must be sufficiently large not to invade the rock, and they must be sufficiently small to form filter cake that filters drill solids and polymers from the filtrate entering the formation. The full-length paper presents a mathematical model that estimates the depth and degree of formation damage caused by solids of different sizes present in drilling or completion fluids. Damage depth and permeability loss are calculated after mud invasion and flowback. Model results agree well with mud filtration experiments.