Lumber Dimensions

Standard Dimensions of Hardwood Lumber

Because of their higher cost and scarcity relative to softwoods, hardwoods are often cut to whatever widths and lengths are most convenient and cost-effective. There is, however, some level of standardization in that boards are cut to standard lengths of 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, and 16 feet. Although standard widths are not established, the hardwood lumber grades do specify minimum widths for each grade as follows:

Hardwood lumber is generally sold in the U. S. in the following thicknesses:

A “quarter” system is commonly used in the hardwood lumber industry when referring to thickness. 4/4 refers to a 1 inch thick board, 6/4 is 1-1/2 inch, 8/4 is 2 inches, and so on. This convention is not normally used for softwood lumber although softwood decking is often marketed as 5/4 thick.

The S1F and S2F thicknesses are for dried lumber. Note that the S1S thicknesses are not standard – the actual thickness is typically subject to contract agreement. The S2S values represent grading standard thickness. That is, what the grader uses for judging the grade of the lumber. In reality, the ACTUAL thickness of planed lumber is generally not 3/16″ under the nominal for 6/4 and less and under 1/4″ for thicker, but will actually be a bit thicker. For example, for 4/4 dried and surfaced there is a lot of 15/16″ hardwood (and also a lot of 3/4″ which is under). There is no industry wide standard for ACTUAL surfaced thickness. Similarly, for green or air-dried 4/4, the grading standard is 1.00″, but most hardwoods are 1/16″ to 1/8″ thicker on the average. The 1.00″ is a minimum and not the actual. [Thanks to wood expert Gene Wengert for providing these details].

Standard Dimensions of Softwood Construction Lumber

Softwood construction lumber is generally machined to lengths that are multiples of 2 feet. The stated length equals the actual length, unlike width and thickness which are typically given in “nominal” dimensions that are somewhat larger than actual dimensions. Width ranges from 2 to 16 inches nominally. Lumber may be categorized according to thickness as follows:

Dimension lumber and boards may be surfaced green or dry, depending on the manufacturer. Timbers are generally surfaced while still green. Green lumber dimensions for thickness and width are closer to the nominal dimensions than the dry lumber dimensions. When a piece of green lumber is surfaced to the “standard” green size, it will shrink to approximately the standard dry size as it dries down to about 15 percent moisture content. (The American Softwood Lumber Standard defines dry as a moisture content of 19 percent or less with an average of 15 percent.) For example, a board that has a nominal thickness of 1 inch has a standard green thickness of 25/32 inch and a standard dry thickness of 3/4 inch. The following table provides an overview of nominal, dry, and green dimensions for softwood construction lumber:

Note: Values under headings “dry” and “green” represent minimum dressed dimensions.

Three moisture-content (MC) levels can be specified for softwood structural lumber: 1) S-Grn or Grn – lumber green or above 19 percent MC, 2) S-Dry or KD 19 – lumber was surfaced at 19 percent MC or less, and 3) MC-15 or KD 15 – lumber was surfaced at 19 percent MC or less. Most structural softwood lumber in the US is KD 19.   The thickness of softwood dimension lumber that is sold with the designation of green or dry (or in between) is the thickness at the time of surfacing and does not change depending on the MC. That is, S-GRN, KD 19, KD 15, etc. all have the same thickness at the time of surfacing. Here is the standard: “When ROUGH (not planed or surfaced) green lumber is specified, the minimum ROUGH green thickness of finish and common boards, dimension and timbers of sizes 1 or more inches nominal thickness shall be not less than 1/8 inch thicker than the corresponding minimum dressed green thickness, except that 20 percent of a shipment may be not less than 3/32 inch thicker than the corresponding minimum dressed green thickness. ” The table above is for green dressed sizes, so this extra thickness cannot be used as it is for rough lumber. Dressed lumber, green or dry, must be the correct thickness at any MC. [Details courtesy of Gene Wengert].