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.