Lumber Dimensions: Nominal vs Actual - Archtoolbox (2024)

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Lumber Dimensions: Nominal vs Actual - Archtoolbox? ›

Softwood lumber that has a thickness over 5" is called timber. While NIST Publication PS20 indicates that nominal 7" to 15" thick timbers should have 3/4" removed for a finished dimension (e.g. a nominal 8x8 timber has a minimum finished size of 7.25" x 7.25"), most timbers over 7" are finished with 1/2" removed.

Nominal vs Actual Lumber Dimensions

The nominal dimensions refer to the rough-sawn size before it is kiln-dried and planed at the sawmill. The 'actual' dimensions refer to the final size that arrives at the lumberyard.

When did lumber dimensions change from actual to nominal? The size of lumber before around 1970 was 3/8″ less than nominal (a 2x4 was 1 5/8″ x 3 5/8″), after around 1970 it became 1/2″ less than nominal (a 2x4 was 1 1/2″ x 3 1/2″). The reason I heard was to get more boards out of a tree.

In North America, the 2x6 size is actually referring to the lumber's “nominal” dimension. A 2x6's real or true dimension is 1.5″ x 5.5″ - within the industry it is referred to as its “actual” size. The nominal size is used when discussing lumber nearly 100% of the time!

Without the rough edges, what went in as a 2-by-4 planks of rough-sawn wood is now a tongue-tripping 1.5-by-3.5, having lost approximately ¼-inch on all sides to the planer and drying processes. “Once upon a time, 2-by-4s really were 2 inches by 4 inches,” Stephens says.

If you see a third number (e.g. 2x4x8), that number is length. Thickness and width are measured in inches, while length is measured in feet. So 2x4x8 is two inches thick by four inches wide, and the board itself is eight feet long.

Now, most timber is milled and planed to give it a little more of a finished look, and a little more of a consistent size and profile. Because of this extra milling, a 2x4 no longer measures a full 2 inches by four inches. Instead, a 2x4 is really only 1 1/2" by 3 1/2". The same is true of pine.

By the 1960s, lumber with a nominal size of 2 x 4 inches was 1-1/2 x 3-1/2 inches in actual finished dimensions.

The lack of resources forced a compromise because thinner 2x4s were a way to compete with these alternatives in the industry. In 1964, size standards, maximum moisture content, and naming were agreed upon. The nominal 2x4 thus became the actual 1-½ x 3-½ inch board.

Why is a 4x4 actually 3.5x3 5? ›

That's just the way lumber is measured. It seems strange, but the reason for this discrepancy is simple, nominal 4 x 4 lumber is actually cut to those measurements from the green, unfinished wood. If interested, you can even find dimensional wood that is unfinished and these measurements will be accurate.

2x Board Sizes

2x4 and 2x6 boards lose 1/2 inch in thickness and 1/2 inch in width before leaving the mill. That means a 2 x 4 board is actually 1-1/2 inches by 3-1/2 inches. The larger 2x boards (2x8, 2x10, etc.) lose 3/4 inch in width.

If your project calls for a piece that is exactly 1 inch thick, you'll want to purchase a 5/4 board and mill it down to the proper size using a jointer/planer.

Why are the nominal dimensions of lumber larger than the actual dimensions? The simple answer is the nominal measurements were derived from a board's size before it has been dried and planed smooth (surfaced) on all 4 sides. The actual measurements are the final dried and surfaced size.

Early concepts called for rough lumber to be of full nominal size, often in the dry condition. After World War I, the increasing demand for construction lumber led to the first national size standard in 1924.

Before it's ready to be sold, dimensional lumber is dried and then planed to make it smooth. Wood shrinks as it dries and the planing or surfacing process removes some of the original material. This means that the wood's actual measurements are now different from the nominal measurements.

Before it's ready to be sold, dimensional lumber is dried and then planed to make it smooth. Wood shrinks as it dries and the planing or surfacing process removes some of the original material. This means that the wood's actual measurements are now different from the nominal measurements.

The difference between nominal dimensions and actual dimensions is that nominal dimensions are the dimensions that are used for the design and/or identification of an object, while actual dimensions are the dimensions that have been measured and/or fabricated.