Nominal size is a measure of the size of a material, object, or structure. It is usually expressed in inches or centimeters. Nominal size is often used to identify objects for purposes of cataloging or ordering.
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Nominal size is a way of expressing dimensions like diameter, length, or thickness. It is distinguished from actual size by its lack of reference to any real world unit of measurement. Rather, nominal size is used (often informally) to indicate rough dimensions. In practice, this means that the actual dimensions of an object labeled with a nominal size may vary considerably from the stated value.
For example, a piece of lumber that is described as being 2″ x 4″ is actually 1.5″ x 3.5″. The nominal width and thickness of the lumber are both twice the actualwidth and thickness. The length will also be twice the actual length plus or minus some small amount depending on the method used to cut the lumber (e.g. 2″ x 4″ lumber can be cut from 8′ long boards).
While nominal sizes are useful for general description, they are not well suited for precise measurements. For this reason, nominal sizes are generally only used in North America, while actual sizes are used in most other parts of the world.
What is Nominal Size?
Nominal size refers to the rough dimensions of a piece of lumber. The nominal measurements are those used for the trade and identify the rough lumber sizes before it is dressed (surfaced, planed or Planed To Thickness, also called S4S) . Dressed lumber is generally smaller than the nominal size by varying amounts depending on the wood species. For instance, a nominal 2×4 actually measures 1-1/2″ x 3-1/2″ when dressed. The actual measurements of any piece of lumber may vary depending on how the lumber is cut from the log, so referring to an undressed board as a 2×4 is really only an approximation of its size.
Examples of Nominal Size
In construction, nominal size generally refers to a measure of the lumber’s rough, pre- machined dimensions before it is finished (surfaced) and cut to length. Examples of lumber nominal sizes would include: 2×4, 2×6, 4×4. The first number always refers to the thickness (called the “nominal thickness”) and the second number refers to the width (called the “nominal width”). The actual finished lumber dimensions will be smaller than the nominal size measures. For example, a “2×4” that is Nominally 8 feet long will usually measure 1-1/2 inches thick by 3-1/2 inches wide and will usually be cut to 6 or 8 feet in length.
Nominal size is also sometimes used when referring to other building materials such as electrical conduit, PVC pipe, metal decking, etc. In these cases, the nominal size may not be an actual measure of the materials finished dimensions but rather a rough pre-machined dimension or even just a name given to a particular size by manufacturers or suppliers. For example, in electrical conduit, there are three common types of conduit named after their interior diameters: ½ inch EMT (electrical metallic tubing), ¾ inch IMC (intermediate metal conduit), and 1 inch rigid metal conduit (RMC). These refer to the interior diameter of each type of conduit before it is machined or threaded on the outside.
Uses for Nominal Size
Nominal size is used in several different situations. One common use is identifying the size of lumber. The lumber industry uses nominal size when referring to the thickness and width of boards. For example, a “two-by-four” board is actually 1 1/2 inches by 3 1/2 inches (or 38 mm × 89 mm). The term “two-by-four” only refers to the rough cut of the lumber, not the finished size.
Another common use for nominal size is in pipe and tubing. In this case, the nominal size is typically different from the actual inside diameter of the pipe or tubing. This can cause confusion, so it’s important to be aware of both the nominal size and the actual inside diameter when ordering or working with pipe and tubing.
Nominal size can also be used in other situations where an exact measurement isn’t necessary or where referencing a general category is more appropriate. For example, clothing sizes are often given as nominal sizes (small, medium, large, etc.), even though there can be significant variation in the actual dimensions of clothing within each category.
How to Measure Nominal Size
Nominal size is a measure of length in which the object being measured is given a name rather than a number. For example, a piece of lumber might be identified as being “two by four” or “four by six.” The term is also used to describe pipes, in which case the nominal size refers to the inside diameter of the pipe.
To measure the nominal size of something, you will need to know its actual dimensions. For instance, a “two by four” piece of lumber is actually 1 1/2 inches by 3 1/2 inches. To measure pipes, you will need to use a tape measure or ruler to measure the inside diameter of the pipe.
Nominal size is a general term used in reference to the approximate dimensions of something. In construction, nominal size usually refers to lumber that is sold in rough cut, unfinished form. The nominal size of a piece of lumber is its rough cut size before it has been planed or trimmed down to its finished dimensions.