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In manufacturing, a nominal size or trade size is a size "in name only" used for identification. The nominal size may not match any dimension of the product, but within the domain of that product the nominal size may correspond to a large number of highly standardized dimensions and tolerances.
For example, dimensional lumber sizes such as "2 by 4" refers to a board whose finished dimensions are closer to 1 1⁄2 inches by 3 1⁄2 inches. A "3 1⁄2-inch" floppy disk's standard dimension is 90 mm, or 3.54 inches, and is advertised to hold "1.44 megabytes" although its capacity is 1,474,560 bytes. A "3⁄4-inch pipe" in the Nominal Pipe Size system has no dimensions that are exactly 0.75 inches. A screw thread has a number of dimensions required to assure proper function but is referred to by a nominal size and a thread design family, for example "1⁄4 inch, 20 threads per inch, Unified National Coarse".


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