Nominal size

In manufacturing, a nominal size or trade size is a size "in name only" used for identification.[1] 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 12 inches by 3 12 inches (1 34 inches by 3 34 inches is typical in the United Kingdom). A "3 12-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 "34-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 "14 inch, 20 threads per inch, Unified National Coarse."

In the United Kingdom, pipe is available that is quoted in both metric size and imperial size. The metric size is larger than the imperial size. For example, both 12 inch and 15 millimetres (0.59 in) copper pipe is actually the same pipe which has a nominal internal diameter of 12 an inch and a nominal external diameter of 15 millimetres[2] (diameter is always internal in the imperial measurement system and always external in metric).

Nominal sizes may be well-standardized across an industry, or may be proprietary to one manufacturer.

Applying the nominal size across domains requires understanding of the size systems in both areas; for example, someone wishing to select a drill bit to clear a "14-inch screw" may consult tables to show the proper drill bit size. Someone wishing to calculate the load capacity of a steel beam would have to consult tables to translate the nominal size of the beam into usable dimensions.

See also


  1. R. K. Rajputpage A textbook of manufacturing technology: (manufacturing processes), Firewall Media, 2008 ISBN 81-318-0244-2 page 705
  2. Nominal pipe sizes

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