Nominal size is defined in ANSI A137.1 Specification for Ceramic Tile as, "the approximate facial size or thickness of tile, expressed in inches or fractions of an inch (or in centimeters or millimeters) for general reference." In other words, nominal size is the generic tile size which is usually noted on the tile box or on product data sheets. Typical nominal sizes are 8"x 8", 12"x 12", 16"x 16", 24" x 24", etc.
This is all about what you can call a tile and not about variation in the box. For example, you can't call 13" tiles 12", but you can call an 11 ¾" tile a 12" tile. The nominal size requirements in ANSI A137.1 limit how much the nominal size can vary from the actual size of the tiles in the box. For example, A137.1 states that for calibrated pressed floor tiles, the tiles can be as small as 3.00% below the nominal size or 3.00% above the nominal size. This does not mean though that you can have tiles 3% above and below nominal size on the same job! The variation in size within a single lot has to do with caliber range which was discussed in the previous blog (click here to learn about caliber range).
Looking at the specification above, for a product that is described as a 12" × 12", the tiles tested from the lot can be as small as 11.64" or as large as 12.36" and still be called 12" x 12".
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Check back, next time we will discuss "What is Warpage?"