Nominal vs Actual Lumber Dimensions: Chart & Explanation

Lumber Production

Why is a two-by-four called a “two-by-four” if it’s not actually two inches by four inches?

Well, when a board is first milled from a log, that green piece of lumber, after that first rough cut, actually does have the approximate width and depth (the nominal lumber dimensions) by which the piece will be referred to from then on. At that point, a two-by-four is, in fact, about two inches by four inches.

The next step in the production process is to either air-dry or kiln-dry the green lumber, in order to reduce the moisture content of the wood. This causes the lumber to shrink as it loses its moisture volume, reducing the actual lumber dimensions. This is done to ensure that when the lumber is finished and sold its dimensions will be stable, and it won’t shrink significantly (which would change the actual lumber dimensions) after somebody uses it.

After being dried, boards are planed (also referred to as “surfaced”), smoothed, and often have their corners slightly rounded. This reduces the actual lumber dimensions even further, and it also lets the producer be sure that the final product has the dimensions required by industry standards. In the case of a two-by-four, those standards specify a finished board with a cross-section of 1.5 inches by 3.5 inches.

Lumber Standards

Although I’m talking about 2x4s here, the same principle applies to other standard sizes, like two-by-sixes, two-by-eights, four-by-fours, and so on. In each instance, the nominal dimension is larger than the actual dimension after drying and planing. Historically, when lumber was rough-sawn and often used green or only partially air-dried, it was closer to the literal 2″ × 4″. But once you factor in moisture reduction through modern kiln-drying (often to around 8% moisture content) and the planing process (commonly known as S4S – surfaced on all four sides), extra material is removed, leading to the modern standardized 1.5″ × 3.5″ finished size.

If you take a look inside older buildings, particularly historic homes or barns, you may find lumber that matches those original rough-sawn dimensions more closely. But lumber standards and building codes have changed since then, with the goal of promoting nationwide uniformity. Having standard lumber sizes goes a long way toward improving the efficiency of the construction industry in general.

Various standards organizations, including the American Lumber Standard Committee (ALSC) of the US Department of Commerce, which produces the American Softwood Lumber Standard in the U.S., and the National Lumber Grades Authority (NLGA) in Canada, make sure that lumber sizes are consistent across different regions. That means a two-by-four purchased in any lumberyard nationwide will measure essentially the same 1.5″ × 3.5″ (within permitted tolerances, of course).

The Terminology is a Convention

Understanding the difference between nominal and actual sizes is important when you’re planning a project. A blueprint might specify a space exactly 3.5 inches wide, and you should know that a so-called “two-by-four” will fit perfectly – despite its nominal dimensions suggesting something else. Over time, builders have learned to rely on these standardized measurements rather than the older nominal labels, (which are still used, largely out of habit and tradition).

The reason we still call a 1.5-inch by 3.5-inch board a “two-by-four” is both historical and practical. The term stuck around from when rough-sawn boards were genuinely closer to two inches by four inches. As milling processes became more refined (removing extra wood to make them smoother and more uniform), the exact measurements changed, but the old naming convention never left. Today, you can walk into any lumberyard, ask for a “two-by-four”, and be sure that you’ll get a piece of lumber that’s 1.5 inches by 3.5 inches in cross-section.

For more about dimensional lumber, see my web page: Dimensional Lumber Weight Calculator & Size Chart.

Guidelines for Converting Nominal to Actual Dimensions

These rules are described in the American Softwood Lumber Standard that I linked to above.

1. Nominal Thicknesses <1 inch

• Subtract 1/8 inch from the nominal dimension to get the actual size.
• Example: A nominal 3/4 × 3/4 board (3/4 inch thick, 3/4 inch wide) ends up measuring 5/8 × 5/8 (3/4 − 1/8 = 5/8).

2. From 1 inch up to <2 inches

• Subtract 1/4 inch from the nominal dimension.
• Example: A nominal 1 × 4 board becomes 3/4 × 3-1/2, meaning you’re removing 1/4 inch from the thickness (while the width follows the rule for dimensions from 2 inches up to 6 inches).

3. From 2 inches up to 6 inches

• Subtract 1/2 inch from the nominal dimension.
• Example: A nominal 2 × 4 has an actual thickness of 1.5 inches (2 − 0.5 = 1.5) and an actual width of 3.5 inches (4 − 0.5 = 3.5).

4. From 7 inches up to 15 inches

• Subtract 3/4 inch from the nominal dimension.
• Example: A nominal 2 × 8 is 1.5 × 7.25 (2 − 0.5 = 1.5 and 8 − 0.75 = 7.25), and a nominal 14 × 14 is 13.25 × 13.25 (14 − 0.75 = 13.25).

5. For any dimension ≥16 inches

• Subtract 1 inch from the nominal dimension.
• Example: A nominal 16 × 16 timber finishes at 15 × 15 (16 − 1 = 15).

Standard Lumber Dimensions Table

About the Author

Jack Gray spent 20 years as a principal roof consultant with the Moriarty Corporation, an award-winning building enclosure consultant firm founded in 1967. Mr. Gray has worked in the roofing industry for over 25 years, with training and practical experience in roof installation, roof inspection, roof safety, roof condition assessment, construction estimating, roof design & specification, quality assurance, roof maintenance & repair, and roof asset management. He was awarded the Registered Roof Observer (RRO) professional credential in 2009. He also served as an infantry paratrooper in the 82nd Airborne Division and has a B.A. from Cornell University.