Dimensional Lumber: Definition, Types, and Sizes

Dimensional Lumber

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Dimensional lumber is wood lumber that is cut to certain pre-defined sizes. This type of lumber is sawn, planed, and sometimes further smoothed to make it immediately ready for many applications. Dimensional lumber is the most common type of lumber used for building because its consistent sizing means that builders can use it for many areas of the home. The term's popularity peaked at the start of the 21st century and its use is now mostly confined to builders and others in the lumber and building trades.

Most Consumer-Level Lumber Is Dimensional

For the average do-it-yourselfer, most lumber that will be used for building is considered to be dimensional lumber. So, it is unnecessary to use the term dimensional lumber when shopping for building materials, because all lumber you will buy at Home Depot, Lowe's, or other home improvement or hardware stores will be already cut to pre-determined sizes. In fact, it is far more difficult to find non-dimensional lumber than it is to find dimensional lumber.

The term came into popular use around the beginning of the 20th century to distinguish lumber cut to pre-determined sizes from other types of wood, such as corded wood for fireplaces. During this period, the building industry sought to control costs and minimize waste by developing standard sizes and grades for building timber. Most significant, though, was the rapid change from the post-and-beam style of house framing to the more efficient balloon framing style. Around 1998, usage of the term dimensional lumber for consumer-level products peaked and then slowly waned.

Nominal Size vs. Actual Size Dimensional Lumber

All dimensional lumber has both nominal and actual dimensions. The most familiar type of dimensional lumber is the two-by-four. Due to the thickness of the saw blade and additional milling processes, the resultant two-by-four product is not 2 inches wide by 4 inches deep. Instead, it is 1 1/2 inches by 3 1/2 inches (38 mm by 89 mm). It can be difficult to remember the difference between nominal and actual dimensions. so the example with the two-by-four may help:

  • Nominal Size: Nominal means "existing in name only." So, a two-by-four that in the physical, real sense is 1 1/2 inches by 3 1/2 inches, is only in name 2 inches by 4 inches. In essence, we are saying we have applied a certain name to the board, but this name does not reflect the board's physical sense.
  • Actual Size: Actual size is as actual and real as if you were using a tape measure or a straight edge to measure the dimensions of the stock.

Dimensional sizes have shifted over the years. This can be perplexing for the do-it-yourselfer removing a wall in a house built before the mid 20th century. For example, many older homes have nominal dimension two-by-fours that also have actual dimensions of 2 inches by 4 inches.

Common Dimensional Lumber Sizes

Softwoods such as Douglas fir, spruce, or Hem-Fir (a mixture of Western Hemlock and AmabilisĀ Fir) used for building framing have the following nominal and actual dimensions. These dimensions do not apply to laminated veneer lumber boards, hardwoods, or floorboards.

  • Nominal: Two-by-four or 2 x 4; Actual Size: 1 1/2 inches x 3 1/2 inches
  • Nominal: Two-by-six or 2 x 6; Actual Size: 1 1/2 inches x 5 1/2 inches
  • Nominal: Two-by-eight or 2 x 8; Actual Size: 1 1/2 inches x 7 1/4 inches
  • Nominal: Two-by-ten or 2 x 10; Actual Size: 1 1/2 inches x 9 1/4 inches
  • Nominal: One-by-two or 1 x 2; Actual Size: 3/4 inch x 1 1/2 inches
  • Nominal: One-by-three or 1 x 3; Actual Size: 3/4 inch x 2 1/2 inches
  • Nominal: One-by-four or 1 x 4; Actual Size: 3/4 inch x 3 1/2 inches

Dimensional Lumber Lengths

In relation to the term dimensional lumber, dimensions are only given to the width and depth. Thus, a dimensional two-by-four board can be 12 feet long, for example, but that length is not figured into the dimensions. Length is treated separately.

Lengths typically are shown as the last number. A typical dimensional lumber designation might read as such: "2 in. x 4 in. x 8 ft." This can be confusing because it is a mixture of nominal and actual sizes. The first two numbers (2 inches and 4 inches) are nominal widths and depths for the lumber. As stated earlier, their actual sizes are 1 1/2 inches and 3 1/2 inches. Yet lengths of dimensional stock are always expressed as actual sizes. So, this example board's actual length is 8 feet.