The PEI rating (ASTM C1027) measures how well a glazed tile's surface resists visible wear from foot traffic; it does not indicate tile strength, water absorption, or slip resistance. Most residential floors perform reliably at Class III, while heavy-traffic areas that collect outdoor grit, such as kitchens, entryways, and hallways, are best suited for Class IV. Higher ratings, such as Class V, are engineered for commercial environments and are rarely necessary for home use. Because PEI only applies to glazed surfaces, unglazed porcelain and natural stone will not feature a PEI class. Once you narrow down the technical rating on a spec sheet, order a physical sample to test how the finish and shade look under your room's specific lighting.