Brackenly

Lumber Dimensions Reference Chart: Nominal vs Actual

This reference page provides the standardized dimensions for softwood and hardwood lumber used in North American construction and woodworking. It details the relationship between nominal size—the industry name used for identification—and actual size, which is the physical measurement of the wood after it has been dried and surfaced (planed). These standards are governed by the National Institute of Standards and Technology (NIST) and the American Lumber Standard Committee (ALSC) for softwoods, and the National Hardwood Lumber Association (NHLA) for hardwoods.

Standard Softwood Dimensions (Dimensional Lumber)

Softwood lumber, including species such as Pine, Fir, Spruce, and Cedar, is typically sold as "dimensional lumber." The nominal size represents the dimensions of the board when it is first cut from a green log. The actual size is the final dimension after the wood has seasoned (dried) and been surfaced on all four sides (S4S). According to the Voluntary Product Standard PS 20-20, the following dimensions are the industry standard for dry softwood lumber.

Nominal Size (Inches) Actual Size (Inches) Actual Size (Metric - mm)
1 x 2 3/4 x 1 1/2 19 x 38
1 x 3 3/4 x 2 1/2 19 x 64
1 x 4 3/4 x 3 1/2 19 x 89
1 x 6 3/4 x 5 1/2 19 x 140
1 x 8 3/4 x 7 1/4 19 x 184
1 x 10 3/4 x 9 1/4 19 x 235
1 x 12 3/4 x 11 1/4 19 x 286
2 x 2 1 1/2 x 1 1/2 38 x 38
2 x 3 1 1/2 x 2 1/2 38 x 64
2 x 4 1 1/2 x 3 1/2 38 x 89
2 x 6 1 1/2 x 5 1/2 38 x 140
2 x 8 1 1/2 x 7 1/4 38 x 184
2 x 10 1 1/2 x 9 1/4 38 x 235
2 x 12 1 1/2 x 11 1/4 38 x 286
4 x 4 3 1/2 x 3 1/2 89 x 89
4 x 6 3 1/2 x 5 1/2 89 x 140
6 x 6 5 1/2 x 5 1/2 140 x 140

Hardwood Lumber Sizing (The Quarter System)

Hardwood lumber (e.g., Oak, Maple, Walnut) is typically sold using the "quarter system" for thickness, where each quarter represents 1/4 inch of thickness in its rough-sawn state. Unlike softwood, hardwood is often sold with random widths and lengths. When hardwoods are surfaced on two sides (S2S), the thickness is reduced as follows, per NHLA grading rules:

Nominal Thickness (Quarters) Rough Thickness (Inches) Surfaced Thickness (S2S)
4/4 1" 13/16"
5/4 1 1/4" 1 1/16"
6/4 1 1/2" 1 5/16"
8/4 2" 1 3/4"
10/4 2 1/2" 2 1/4"
12/4 3" 2 3/4"

Standard Lengths and Tolerances

Standard lengths for softwood lumber are typically produced in 2-foot increments, starting at 6 feet and extending up to 24 feet or longer for specialized structural members. According to ALSC PS 20-20 Section 5.3, lumber must be trimmed to the specified length with a tolerance for "over-length" but generally no allowance for "under-length" in structural grades. For example, a board labeled as 8 feet must be at least 96 inches long, though it may be slightly longer to allow for end-squaring by the end user.

Historical Context of Lumber Standardization

The discrepancy between nominal and actual sizes originated in the late 19th and early 20th centuries. Historically, a 2x4 was cut to exactly 2 inches by 4 inches in its green, rough state. As the lumber industry moved from local sawmills to national distribution via rail, mills began planing (surfacing) the wood to reduce weight for shipping and to ensure uniform dimensions for builders. The first national size standard was established in 1924, with major revisions in 1964 and the adoption of the current PS 20 standard in 1970, which officially linked actual dimensions to moisture content to ensure stability in construction.

Last verified: 2026-06-28

Sources

Related Articles