Nominal size

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 1/2 inches by 3 1/2 inches. A "3 1/2 inch" floppy disk 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".

A nominal size may not even carry any unit of measure, for example 120 film is a film format made by different manufacturers, interchangeable in cameras but with no particular correlation to "120" in either inch or millimetre units. American wire gauge is a series of sizes for copper wire, with an arbitrary but well-standardized relationship between the size number and the dimensions of the finished wire.

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 "1/4 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 8131802442 page 705