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
(1 3⁄4 inches by 3 3⁄4 inches is typical
in the United Kingdom). 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."
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 1⁄2 inch and 15 millimetres
(0.59 in) copper pipe is actually the same pipe which has a
nominal internal diameter of 1⁄2 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 "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.
When considering the engineering tolerance between a shaft (or bolt)
going through a hole in some other part (such as a nut), both the
shaft (or bolt) have the same nominal size (also called the basic
size),[3][4][5] but all the holes are physically larger and all the
shafts are physically smaller in order that any shaft (or bolt) of a
given nominal size can fit into any hole of the same nominal size.
See also[edit]

Paper size
Trade gallon
US standard clothing size
Caliber
Preferred number
Real versus nominal value
Nominal impedance

References[edit]

^ R. K. Rajputpage A textbook of manufacturing technology:
(manufacturing processes), Firewall Media, 2008
ISBN 81-318-0244-2 page 705
^ http://www.engineeringtoolbox.com/nps-nominal-pipe-sizes-d_45.html
Nominal pipe sizes
^ Coban Engineering. "Tolerancing Definitions".
^ Bryan R. Fischer. "Mechanical Tolerance Stackup and Analysis, Second
Edition". 2011. p. 410.
^ Bruce J. Black "Workshop Processes, Practices and Materials". 2015.
p. 70.

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