What is a Nominal Size?

A Nominal Size is a measurement of a product or object that states its dimensions in name only. The real measurement may be slightly different, but it is close enough to be called that name.

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Nominal size is a general term used to describe the size of something. In the case of lumber, a board is considered a certain nominal size when it is cut to a specific length and width but its exact measurements may be slightly larger or smaller. This is in contrast to actual size, which refers to the exact measurements of a board.

Lumber is commonly sold in two different categories: dimensional lumber and finished lumber. Dimensional lumber is raw lumber that has been cut to standard widths and lengths and is used in construction. Finished lumber has been planed and sanded smooth and is used in projects such as furniture or cabinets. Both dimensional and finished lumber can be found in nominal sizes.

What is a Nominal Size?

A nominal size is a size that is used for identification purposes only. NominalSize is typically used to identify pipe sizes, lumber dimensions, and thread sizes. For example, a 2-inch nominal sized pipe has an outside diameter of 2-1/8 inches and an inside diameter of 1-7/8 inches. It is important to note that dimensional lumber (the actual size) will always be smaller than the nominal size by about 1/8 inches in thickness and width. This difference exists because the actual lumber dimensions include the thickness of the boards before they are planed smooth (surfaced) on all four sides.

Why is it Important?

Nominal size is important in many fields, including business, engineering, and construction. In general, it is a way of standardizing measures and allowing for easy comparison between different products or materials. For example, in construction, two by four lumber is always referring to lumber that is actually 1.5 inches by 3.5 inches (nominal measure). This allows for easy comparison between different types of lumber when the actual measurements may vary slightly. In business, nominal size can be used to compare the relative sizes of companies.

How to Use Nominal Size?

Nominal size is used to describe the rough dimensions of a piece of lumber before it has been planed (smoothed) or surfaced. The nominal size is always greater than the finished ( actual ) size. For example, a 2-by-4 board starts out as a 2-by-4 board, but after it has been planed and/or surfaced, it becomes 1 1/2 inches by 3 1/2 inches.

To find the actual size of lumber, simply subtract 1/2 inch from the nominal width and add 1/2 inch to the nominal thickness. For example, a 2-by-4 board is actually 1 1/2 inches by 3 1/2 inches when finished.

When shopping for lumber, you might notice that the dimensions are listed in both nominal size and actual size (for example, 2x4s are available in both 2-inch and 1 1/2 inch versions). Be sure to check the actual size before making your purchase!


In conclusion, a “nominal size” is a general size category that a product falls into. The true dimensions of the product may be slightly different, but they will be close to the average size in that category. For example, a 2×4 board is actually 1.5 inches by 3.5 inches, but it is called a “2×4” because that is the average size of boards in that category. Nominal sizes are used for convenience and are not intended to be exact measurements.