Guide
Lumber Dimensions: Nominal vs Actual
Published July 19, 2026
A 2x4 is not 2 inches by 4 inches. It measures 1-1/2 by 3-1/2 inches, and every calculation that treats the nominal name as the real dimension is wrong by a meaningful margin — an inch and a half of missing width per stud adds up fast across a wall, and a header sized against the wrong depth is a structural error, not a rounding one.
The gap between the name and the size isn't an accident or a rounding convention picked at random. It comes from how dimensional lumber is actually made, and understanding that process is what makes the surfaced-size table below easy to remember instead of a list to look up every time.
Why a 2x4 is not 2 inches by 4 inches
A board's nominal size names the rough-sawn dimension it started at, cut directly from the log before any further processing. From there, the board is kiln-dried, which shrinks it as moisture leaves the wood, and then surfaced on all four faces — planed smooth, which removes additional material specifically to produce a flat, consistent, splinter-free finished piece. Drying and surfacing together remove roughly 1/2 inch from each nominal dimension under 8 inches, which is why a rough-sawn 2x4 becomes a dressed 1-1/2 by 3-1/2, and a rough-sawn 1x6 becomes a dressed 3/4 by 5-1/2.
The naming convention stuck because it describes what a mill cuts from a log, which is the unit the sawmill actually plans production around, while the dressed size describes what a carpenter actually gets. Both numbers are real; they just describe two different stages of the same board.
The dressed-size table
- 1x2 — nominal 1 x 2 in, dressed 3/4 x 1-1/2 in.
- 1x4 — nominal 1 x 4 in, dressed 3/4 x 3-1/2 in.
- 1x6 — nominal 1 x 6 in, dressed 3/4 x 5-1/2 in.
- 2x4 — nominal 2 x 4 in, dressed 1-1/2 x 3-1/2 in.
- 2x6 — nominal 2 x 6 in, dressed 1-1/2 x 5-1/2 in.
- 2x8 — nominal 2 x 8 in, dressed 1-1/2 x 7-1/4 in.
- 2x10 — nominal 2 x 10 in, dressed 1-1/2 x 9-1/4 in.
- 2x12 — nominal 2 x 12 in, dressed 1-1/2 x 11-1/4 in.
- 4x4 — nominal 4 x 4 in, dressed 3-1/2 x 3-1/2 in.
- 6x6 — nominal 6 x 6 in, dressed 5-1/2 x 5-1/2 in.
Notice the deduction grows past a nominal 6 inches: widths under 8 inches lose 1/2 inch, but 8, 10 and 12 inch nominal widths lose 3/4 inch instead of 1/2. There isn't a single clean rule that covers every width — the table is the actual reference, not a formula to reconstruct from memory.
Where the nominal size still matters
Framing calculations should always use the dressed size — stud, floor joist, rafter and header spans, deflection and load figures are all engineered against the actual cross-section a board provides, not its rough-cut ancestor. Using 4 inches instead of 3-1/2 for a joist depth in a deflection calculation, for instance, meaningfully overstates the joist's stiffness.
Pricing and sales, by contrast, run on the nominal size by long-standing convention: lumber is quoted and sold as '2x4' or '2x6' stock, and board-foot pricing is calculated from the nominal dimension even though the actual dressed board is smaller. Board feet explained covers that pricing convention in full, including why it persists even though it technically overstates the wood you're actually buying.
Framing spacing and the module the dressed size creates
The 1-1/2 inch dressed thickness of standard framing lumber isn't incidental to on-center spacing — it's why 16 inch on-center framing lands a sheathing panel edge exactly at the centerline of a stud rather than somewhere in open air. A 4 foot wide sheet divides evenly into three 16 inch bays, and the stud's dressed width sits centered on each on-center mark, giving the sheet's edge roughly 3/4 inch of solid bearing on either side of the line. Get the actual width wrong in your head and that bearing margin looks larger or smaller than it really is, which matters when you're deciding how close to the edge a fastener can go.
Metric and engineered lumber break the pattern
Engineered lumber — LVL, glulam, I-joists — is generally sold and specified at its true, actual dimension rather than a nominal one, because it is manufactured to a target size rather than sawn and dried down to it. A 1-3/4 by 11-7/8 inch LVL is exactly that, with no separate nominal name to translate from, which is one of the quieter advantages of designing with engineered members: the number on the spec sheet is the number in your hand.
Metric-dimensioned lumber, more common outside North America, is sized in millimeters and generally does not line up with either the nominal or the dressed inch figures above — a 38 x 89 mm stud, common in Canadian and some international framing, is close to but not identical to a dressed 2x4's 1-1/2 by 3-1/2 inches. Working in metric and imperial on the same job goes further into that particular near-match: close enough to assume, wrong enough to matter.
FAQ
Does a 2x4 always measure exactly 1-1/2 by 3-1/2 inches?
Very close to it, with a small manufacturing tolerance, and it holds across mills and regions because it's a widely followed sizing convention for surfaced dimensional lumber. Rough-sawn or green (undried) lumber, which is less common at a retail yard, can measure closer to its full nominal size.
Why do 2x8, 2x10 and 2x12 lose 3/4 inch instead of the 1/2 inch that 2x4 and 2x6 lose?
Wider boards are cut from wetter, less dimensionally stable sections of the log and shrink more during kiln drying, so the surfacing pass removes more material to reach a flat, consistent finished face. It's a real physical difference in how much the wider stock moves while drying, not an arbitrary step in the table.
Should I use nominal or dressed size when ordering lumber for a project?
Order by the nominal name — '2x6, 10 foot' is how yards stock and price it — but do every span, spacing and load calculation using the dressed dimension, since that's the actual material a board provides once it's in your hands.
Do plywood and OSB follow the same nominal-versus-actual pattern as dimensional lumber?
Their thickness does something similar — a nominal 3/4 inch panel is commonly closer to 23/32 inch actual — but their face dimensions generally don't; a 4x8 sheet is genuinely close to 48 by 96 inches. See sheet goods: sizes, coverage and the tongue for the panel-thickness figures.
Is the nominal-to-actual gap the same in every country?
No — it's a North American surfaced-lumber convention. Lumber sized and sold in millimeters elsewhere doesn't use a nominal-versus-dressed naming split the same way, which is one of several reasons a metric stud and an imperial stud aren't interchangeable even when their sizes look close on paper.