Lumber Size Chart: Nominal vs Actual

Softwood framing lumber, surfaced four sides (S4S), dry. These are the actual dimensions you build to.

The pattern is worth internalising rather than looking up: subtract ½ in up to 6 in nominal, subtract ¾ in from 8 in nominal and above. Thickness follows the same rule — nominal 1 in is ¾, nominal 2 in is 1½, nominal 4 in is 3½.

Length is the exception. A 2x4 sold as 8 ft is 8 ft, or occasionally 92⅝ in if it is a precut stud, which is sized so that with a single bottom plate and a doubled top plate the wall finishes at 8 ft 1⅛ in — enough for drywall to clear the floor.

It used to be. Rough-sawn green lumber came off the mill at a full 2 x 4 in, and that is what got shipped.

Two things shrank it. First, planing: running the board through a planer to make it straight, smooth and consistent takes material off all four faces. Second, drying: wood shrinks across the grain as its moisture content drops, and lumber is now surfaced dry, at 19% moisture or less, rather than green. Kiln-dried and planed, a nominal 2x4 lands at 1½ x 3½.

The sizes were standardised through the twentieth century, tightened again in 1964, and are now set by the American Softwood Lumber Standard PS 20. The nominal number stuck because the trade had a hundred years of habit and every span table, joist layout and stud spacing convention was built on it.

You will see the older sizes in old houses. Pull the trim in a 1920s house and the studs are often a full 2 x 4, or something in between if it was surfaced to a different standard. That matters when you sister a new joist to an old one and find a ¼ in step in the middle of the floor.

Rough-sawn and green lumber still exists — timber-frame stock, barn material, some hardwood — and there the number on the ticket may be the real dimension. Measure it rather than assuming.

Panels are more honest about length and width, and less honest about thickness.

Sheets are a true 48 x 96 in, except tongue-and-groove subfloor, which is 47½ in wide on the face so that the joint lands on a joist and the sheets still lay out on 16 in and 24 in centres.

The thickness gap catches people on two jobs in particular. Cabinet and shop work, where a ¾ in dado cut to a nominal ¾ in leaves a sloppy joint — cut to the sheet, not the number. And subfloor patching, where a nominal ½ in OSB patch into a 15/32 in plywood floor gives you a hump under the vinyl.

Board feet are priced on nominal, measured on nominal. A board foot is 144 cubic inches computed from nominal thickness and width by actual length: thickness x width x length ÷ 12 with thickness and width in inches and length in feet. A 2x8 at 10 ft is 13.33 board feet even though the actual section is 1½ x 7¼, which is 10.9 board feet of wood. You are billed for the sawn size, not the surfaced one, and that is the convention throughout the industry. Use actual dimensions in a board-foot calculation and you will under-price the order by roughly a fifth.

Hardwood is the same idea in different clothing. It is sold in quarters of an inch of rough thickness — 4/4 is one inch rough, which surfaces to about 13/16, and 8/4 is two inch rough surfacing to about 1¾. You pay for 4/4 and you receive 13/16.

Framing layout uses actual. A wall of 2x4 studs at 16 in on centre gives you a 14½ in cavity between studs, which is why batt insulation is made 15 in wide and why an outlet box has to fit that gap. Cavity depth is 3½ in, and that is what governs the R-value you can get into a 2x4 wall — nominal thinking here gives you an insulation order that does not fit.

Header and beam depths are actual. Two 2x10s with ½ in plywood between make a 3½ in wide by 9¼ in deep header, which matches a 2x4 wall exactly. That coincidence is the reason the built-up header exists in that form.

Concrete forms and openings are actual. Frame a rough opening off nominal numbers and every door in the house will be off by an inch and a quarter.

The habit that avoids all of it: talk in nominal when you are ordering and pricing, and switch to actual the moment you are laying anything out. They are two different conversations about the same board.