Dimensional lumber is the structural element responsible for nearly every type of residential home being built today. Dimensional lumber is the most widely used structural material in the U.S. and Canada's home-building industry and almost everyone relies on dimensional lumber.
Dimensional lumber is wood lumber that is cut to pre-defined, standard sizes. Dimensional lumber sizes refer to depth and width, not length.
Dimensional lumber is sawn, planed, and sometimes further smoothed to make it immediately ready for many applications. It is the most common type of lumber used for building because its consistent sizing allows builders to use it interchangeably throughout the home.
Standardization also means that builders or do-it-yourselfers throughout the entire country are using lumber that is sized the same way. A two-by-four with an actual measurement of 1 1/2 inches by 3 1/2 inches in one state will be exactly the same as a two-by-four across the country. The modern home would not be possible without dimensional lumber.
Lumber that is cut to standard widths and depths, but never lengths.
Typical dimensional lumber: two-by-fours and four-by-fours.
Sheet goods are not considered to be dimensional lumber.
Nominal and actual dimensions differ except in some older homes.
Nominal dimension is the name of the lumber: two-by-four, for example.
Actual dimension is the size: 1 1/2 inches by 3 1/2 inches, for example
Dimensional Lumber and Modern Building
The term dimensional lumber 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.
For the average do-it-yourselfer, most long, thin lumber that will be used for building is considered to be dimensional lumber. Other types of lumber, such as plywood and other sheet goods, do come in prescribed standard sizes but are not referred to as dimensional lumber.
It is rarely necessary to use the term dimensional lumber when shopping for building materials. 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.
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 by 4 inches. Instead, it is 1 1/2 inches by 3 1/2 inches.
Tip
One rule of thumb for quickly calculating actual lumber width: subtract 1/2-inch from the nominal width on all except for 1-inch nominal width boards. For 1-inch nominal boards, subtract 1/4-inch.
It can be difficult to remember the difference between nominal and actual dimensions. Even experienced woodworkers or do-it-yourselfers may find themselves struggling to remember the true width of a two-by-ten. As a rule of thumb:
Nominal Lumber Sizes
Nominal is the giveaway and an easy way to remember this one. Nominal means name, or in this sense, 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 nickname to the board, but this nickname does not accurately describe the board's physical sense.
Actual Lumber Sizes
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. Actual size is anywhere from 1/4 inch to 3/4 inch less than nominal size.
Common Dimensional Lumber Sizes
Softwoods such as Douglas fir, spruce, or Hem-Fir (a mixture of Western Hemlock and Amabilis 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.
Dimensional Lumber: Nominal Size vs. Actual Size
Nominal Size Actual Size
Two-by-four or 2 x 4 1 1/2 inches x 3 1/2 inches
Two-by-six or 2 x 6 1 1/2 inches x 5 1/2 inches
Two-by-eight or 2 x 8 1 1/2 inches x 7 1/4 inches
Two-by-ten or 2 x 10 1 1/2 inches x 9 1/4 inches
One-by-two or 1 x 2 3/4-inch x 1 1/2 inches
One-by-three or 1 x 3 3/4-inch x 2 1/2 inches
One-by-four or 1 x 4 3/4-inch x 3 1/2 inches
Dimensional Lumber Lengths
In relation to the term dimensional lumber, dimensions only refer 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. The 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. When you take that "2 in. x 4 in. x 8 ft." board home, it will actually measure out to be 1 1/2 inches by 3 1/2 inches by 8 feet.
When Nominal and Actual Dimensions Match
In rare cases, nominal and actual dimensions are the same. Anyone tearing down an especially old home may have encountered this before.
Actual 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 are actually 2 inches by 4 inches. It is difficult to reuse such stock for other applications around the house.
A prime example of this difficulty involves walls built with current-day nominal two-by-four studs that are actually 1 1/2 inches by 3 1/2 inches. Repairing or sistering that wall system with older two-by-fours that are actually 2 inches by 4 inches would cause portions of the wall to protrude by an extra half-inch.
