Lumber actual size chart: nominal vs actual dimensions

Because moisture and grade change the real size after the mill. A dry 2x12 is dressed to 11 1/4 in, a green one to 11 1/2 in, and both keep shrinking as they dry, about 1% of width for every 4 points of moisture lost. On a framing forum, rafters from one order measured near 11 1/8 in in a dry No. 1 grade and almost 11 5/8 in in a green grade, which left uneven eaves. Buy one grade and one stamp for a run, check the S-DRY or S-GRN mark, and crown and sort pieces by width before you nail them up.

Treated lumber is soaked in preservative after it is cut and is often sold wet, so its size depends on how much it has dried since. Two boards from the same pile can differ by 1/8 in or more, and they keep shrinking on the deck. Builders on a forum describe sorting treated decking into piles of similar width and laying it in order so the joints do not step, and checking every 2x8 instead of assuming 7 1/4 in. Use the green column as the upper bound and measure the actual boards you have before you cut anything that must fit.

Not in the same run, because the depths differ. Common I-joists are 11 7/8 in deep while a dry 2x12 is 11 1/4 in, a 5/8 in gap. A building-science forum warns that mixing dimensional lumber and I-joists, especially as rim board, can cause real trouble; I-joist makers sell matching rim board instead. If a floor must combine them, follow the I-joist maker's details. For which lumber size a span needs, use the floor joist span calculator rather than the size chart, since the chart only gives dimensions.

A dry 2x4 is 1 1/2 × 3 1/2 in (38 × 89 mm); green it is 1 9/16 × 3 9/16 in. Why and how it got that way is in actual size of a 2x4.

A 4x4 is 3 1/2 × 3 1/2 in and a 6x6 is 5 1/2 × 5 1/2 in. A 4x4 counts as dimension lumber, so it loses 1/2 in per face, and a 6x6 is a timber, which also loses 1/2 in whether dressed dry or green. Treated posts are often a little over size while wet, up to about 3 9/16 in for a 4x4. Rough-sawn posts from a local sawmill can be full size, a true 4 × 4 or 6 × 6 in. Measure the post before you buy brackets or post bases, since a full-size post will not fit hardware made for 3 1/2 in.

Because PS 20 takes 3/4 in off widths of 8 in and more, and only 1/2 in off narrower widths. So a 2x6 is 5 1/2 in but a 2x8 is 7 1/4 in, a 2x10 is 9 1/4 in and a 2x12 is 11 1/4 in. Wider boards shrink and cup more across their width, so they are sawn and planed with a larger allowance. The green sizes keep the pattern: a green 2x8 is 7 1/2 in. Planning a stair or a stacked layout from 7 1/2 in will come up short by 1/4 in per board, a common framing mistake.

Often close to it, but there is no national rough size. Rough lumber has not been planed, so a local sawmill may cut a 2x4 at a true 2 × 4 in, or a little over to allow for drying. Older houses can hold full-size rough lumber, or boards dressed before today's sizes were set that run larger than the chart, which is why repair work often needs shims or ripped stock. Measure rough stock board by board, and expect it to shrink as it dries.

It means five quarters of an inch, the nominal thickness 1 1/4 in. In softwood, a dry 5/4 board is dressed to 1 in under PS 20, and a 6/4 board to 1 1/4 in; a 5/4 × 6 deck board is 1 × 5 1/2 in. In hardwood the quarter names the rough thickness, so a 5/4 hardwood board is 1 1/4 in rough and about 1 1/16 in once surfaced on two sides. Board feet for both are counted on the nominal or rough quarter, not the planed thickness, so a 5/4 board tallies as 1.25 in thick.

Usually even lengths from 8 ft to 16 ft, with 18, 20 and sometimes 24 ft in dimension lumber. PS 20 sets standard lengths in multiples of 1 or 2 ft as the grading rules say, and lumber 2 in thick or less is trimmed to at least its nominal length and no more than 3 in over. So an 8 ft 2x4 is between 96 and 99 in. Studs are also sold precut at 92 5/8 in for 8 ft ceilings and 104 5/8 in for 9 ft ceilings, so a wall with three plates comes out just over 97 in or 109 in.

They tell you the moisture content when the piece was surfaced. S-DRY and KD mean dry, at 19% moisture or less, so the dry column of the chart applies. MC 15 or KD 15 means dried to 15% under some grading rules, with the same dry sizes and less shrinkage still to come. S-GRN means surfaced green, above 19%, so the piece is dressed to the larger green size and will shrink. HT means heat treated and says nothing about size. The rest of the stamp gives the grading agency, mill, species and grade.

PS 20 lists its own metric minimums: a 2x4 is 38 × 89 mm, a 2x6 is 38 × 140 mm, a 2x8 is 38 × 184 mm, a 2x10 is 38 × 235 mm and a 2x12 is 38 × 286 mm. A 1x board is 19 mm thick and a 4x4 is 89 × 89 mm. Converting the inch sizes exactly gives slightly higher figures, 38.1 × 88.9 mm for a 2x4, because the standard rounds. European metric framing timber is made to different sizes, such as 47 × 100 mm, so it is not a direct substitute for US dimension lumber.

For dried lumber, PS 20 counts shrinkage of 1% of the size for each 4 percentage points of moisture lost below the level it was surfaced at, 0.7% for redwood and the cedars. A 2x10 surfaced at 19% that dries to 11% in a heated house can lose about 2% of its 9 1/4 in width, close to 3/16 in. Green lumber shrinks much more, which is why it is surfaced larger. Wood shrinks most across the growth rings and very little along its length, so lengths barely change. Let lumber acclimate before trim work or tight joints.

Usually not. Structural panels are sold by a Performance Category, which is the nominal thickness used in building codes, and the panel itself is labeled with a slightly thinner decimal or fractional thickness. A 3/4 in panel is commonly 23/32 in and a 1/2 in panel 15/32 in. Sanded cabinet-grade plywood can measure differently again. Check the label or measure with calipers before you cut dadoes or rabbets for it. Sheet counts for walls, floors and roofs are in the plywood sheet calculator rather than this lumber chart.

No. PS 20 covers softwood only, so its dry and green tables do not apply to oak, maple or walnut. Hardwood is sold rough by the quarter inch, 4/4, 5/4, 8/4 and so on, in random widths and lengths, and the yard may surface it for you. A 4/4 board surfaced two sides is about 13/16 in thick and an 8/4 about 1 3/4 in. Widths are whatever the board is, measured to the nearest inch for the tally. Hardwood board feet use the rough thickness, which the board foot calculator handles with its quarters setting.