Tiles are graded using a numerical numbering system based on their thickness and quality. A rating of 1 is the highest quality and thickest tile available, 3/4-inch thick, and you can use them anywhere. Grade 2 reflects that some imperfections exist but the tile still is usable on walls or floors. These tiles are approximately 1/2-inch thick. Grade 3 tiles are the thinnest, 1/4-inch thick, and are appropriate for use only on walls. Grades 1 through 3 sometimes are referred to as standard, secondary or cull grade, respectively.
The Porcelain Enamel Institute rates tiles on their ability to resist abrasions caused by everyday wear and tear. The higher the roman numeral beside "PEI" in the printed rating, the better the resistance. Tiles rated I or II are not suited for installation on a floor. Floor tiles must be rated at III or higher for residential use; the highest rating, IV, is recommended for high-traffic commercial floors. The PEI rating applies only to glazed tiles.
A tile's water absorption rating helps you determine if the tile is suitable for installation in your room. The rating is calculated based on how much water the tile absorbs in relation to its weight. For example, tiles rated as nonvitreous will not survive a high-moisture environment such as a shower because they absorb more than 7 percent of their weight. For wet areas, including exterior patios, the tiles must be vitreous or impervious. This test is completed by exposing the tile to dyed, standing water for 16 hours and then checking for penetration.