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

Lumber

Lumber or timber is wood in any of its stages from felling through readiness for use as structural material for construction, or wood pulp for paper production....

 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

Floppy disk

A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...

 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

Nominal Pipe Size

Nominal Pipe Size is a North American set of standard sizes for pipes used for high or low pressures and temperatures. Pipe size is specified with two non-dimensional numbers: a nominal pipe size for diameter based on inches, and a schedule for wall thickness...

 system has no dimensions that are exactly 0.75 inches. A screw thread

Screw thread

A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered 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

120 film

120 is a film format for still photography introduced by Kodak for their Brownie No. 2 in 1901. It was originally intended for amateur photography but was later superseded in this role by 135 film...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

American wire gauge

American wire gauge , also known as the Brown & Sharpe wire gauge, is a standardized wire gauge system used since 1857 predominantly in the United States and Canada for the diameters of round, solid, nonferrous, electrically conducting wire...

 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

Drill bit sizes

Drill bits are the cutting tools of drilling machines. They can be made in any size to order, but standards organizations have defined sets of sizes that are produced routinely by drill bit manufacturers and stocked by distributors....

. 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

• Paper size
  Paper size
  Many paper size standards conventions have existed at different times and in different countries. Today there is one widespread international ISO standard and a localised standard used in North America . The paper sizes affect writing paper, stationery, cards, and some printed documents...
  
  
• Trade gallon
  Trade gallon
  A Trade gallon is a term used to denote the sizes of standard plant containers in horticultural industries. A trade gallon is equal to approximately .71 U.S. liquid gallons, and it should not be confused with the British Imperial gallon or the British grain gallon...
  
  
• US standard clothing size
  US standard clothing size
  US standard clothing sizes were developed from statistical data in the 1940s-1950s. They were similar in concept to the EN 13402 European clothing size standard....
  
  
• Caliber
  Caliber
  In guns including firearms, caliber or calibre is the approximate internal diameter of the barrel in relation to the diameter of the projectile used in it....
  
  
• Preferred number
  Preferred number
  In industrial design, preferred numbers are standard guidelines for choosing exact product dimensions within a given set of constraints....
  
  
• Battery nomenclature
  Battery nomenclature
  Standard battery nomenclature describes portable dry cell batteries that are interchangeable in physical dimensions and electrical characteristics between manufacturers. The long history of disposable dry cells means that many different manufacturer-specific and national standards were used to...
  
  
• Real versus nominal value
  Real versus nominal value
  In economics, nominal value refers to a value expressed in money terms in a given year or series of years. By contrast, real value adjusts nominal value to remove effects of price changes over time...