Lumber Nominal vs Actual
Actual finished dimensions for 1×, 2×, 4×, 6× lumber — inches and millimetres.
How to use
Pick a nominal lumber size from the dropdown. The calculator returns the actual finished (S4S, dressed) dimensions in inches and millimetres.
- Select the nominal size (the size printed on the lumber tag at the yard — 2×4, 1×6, 4×4, etc.).
- Read the actual width and actual thickness — these are the dimensions of the lumber after surfacing.
- The full reference table below shows every common nominal size with its S4S actual dimensions.
- Hardwood and rough-sawn lumber follow different conventions — this calculator covers North American softwood S4S only.
Reviewed 3 June 2026 · methodology cited
Why nominal and actual differ
North American softwood lumber is sold by a nominal size that is bigger than the wood you actually take home. A 2×4 stud is sold as "two by four" but measures 1½ by 3½ inches after the mill has surfaced and dressed it. A 1×6 board is ¾ by 5½ inches. The nominal size matches the rough-sawn dimension before drying and planing; the actual size is what survives the lumberyard's S4S (Surfaced Four Sides) process.
This calculator and its reference table show the actual S4S dimensions for every common nominal size from 1×2 through 6×12. Use it when stacking framing for a wall height, sizing a header that needs to fit between studs, calculating drywall edge gap, or figuring out why your cabinet face frame is exactly half an inch short.
The surfacing tolerance
The surfacing tolerance is standardized in PS 20 (the US Voluntary Product Standard for American Softwood Lumber). For dimension lumber up to 1 inch nominal thickness, the actual is nominal minus ¼ inch. For 1¼ to 1½ inch nominal (rare), the actual is nominal minus ⅜ inch. From 2 inches up to 4 inches nominal, the actual is nominal minus ½ inch. Above 4 inches, the actual is nominal minus ¾ inch.
The same rule applies to width up to 6 inches nominal (subtract ½ inch); from 7 inches up to 16 inches nominal, subtract ¾ inch. So a 2×10 is actually 1½ by 9¼ inches; a 2×12 is 1½ by 11¼ inches; a 4×4 is 3½ by 3½ inches; a 6×6 is 5½ by 5½ inches. These are the dimensions on which every span table and structural calculator is based — the nominal size is purely a name.
Full reference table
| Nominal | Thickness | Width | mm | mm |
|---|---|---|---|---|
| 1×2 | 3/4″ | 1 1/2″ | 19 | 38 |
| 1×3 | 3/4″ | 2 1/2″ | 19 | 64 |
| 1×4 | 3/4″ | 3 1/2″ | 19 | 89 |
| 1×6 | 3/4″ | 5 1/2″ | 19 | 140 |
| 1×8 | 3/4″ | 7 1/4″ | 19 | 184 |
| 1×10 | 3/4″ | 9 1/4″ | 19 | 235 |
| 1×12 | 3/4″ | 11 1/4″ | 19 | 286 |
| 2×2 | 1 1/2″ | 1 1/2″ | 38 | 38 |
| 2×3 | 1 1/2″ | 2 1/2″ | 38 | 64 |
| 2×4 | 1 1/2″ | 3 1/2″ | 38 | 89 |
| 2×6 | 1 1/2″ | 5 1/2″ | 38 | 140 |
| 2×8 | 1 1/2″ | 7 1/4″ | 38 | 184 |
| 2×10 | 1 1/2″ | 9 1/4″ | 38 | 235 |
| 2×12 | 1 1/2″ | 11 1/4″ | 38 | 286 |
| 4×4 | 3 1/2″ | 3 1/2″ | 89 | 89 |
| 4×6 | 3 1/2″ | 5 1/2″ | 89 | 140 |
| 4×8 | 3 1/2″ | 7 1/4″ | 89 | 184 |
| 6×6 | 5 1/2″ | 5 1/2″ | 140 | 140 |
| 6×8 | 5 1/2″ | 7 1/4″ | 140 | 184 |
| 6×12 | 5 1/2″ | 11 1/4″ | 140 | 286 |
Notes for framing and stacking
Wall framing uses the actual width when figuring sheathing courses. A wall framed with 2×6 studs is 5½ inches thick, not six — your jack studs, kings, headers, and sole plate all stack to the same actual thickness, and your sheathing and drywall sit flush on a 5½-inch face. Sole plates and top plates use the same actual lumber, so doubled top plates add 3 inches to wall height (two times 1½ inches), not four.
Headers built up from two 2× members with a ½-inch plywood spacer between them measure 3½ inches thick — the same width as a 2×4 wall. Headers in 2×6 walls need a 3½-inch deep spacer or a triple 2× to fill the 5½-inch width. Joist hangers are sized to actual width: a hanger for a "2×8" joist is sized for 1½-inch nominal width and 7¼-inch nominal depth. Always check the actual dimension on the hanger spec sheet against the actual dimension of your joist before specifying.
Frequently asked questions
Why does a 2×4 not measure 2 by 4 inches?
A "2×4" is rough-sawn at the mill at approximately 2 by 4 inches, then dried, planed, and surfaced to remove imperfections and produce a smooth, uniform finished product. The dressing process removes about ¼ inch from each face, so the final dimensions are 1½ by 3½ inches. The original rough-sawn name stayed as the sales description.
Does this apply to hardwood lumber?
No. Hardwood is sold by the quarter system (4/4, 5/4, 8/4 for nominal thickness in quarters of an inch) and is usually sold rough — you specify whether you want it surfaced two sides (S2S), surfaced three sides (S3S), or four sides (S4S), and the actual thickness depends on how much you remove. This calculator is for North American softwood S4S dimension lumber.
Is the 2×4 size standardized globally?
Roughly, but not exactly. The nominal-to-actual relationship for North American dimension lumber is set by US PS 20 (American Softwood Lumber Standard) and Canadian Standards Association CSA O141. European, Australian, and other markets use direct metric dimensions (e.g. 38×89 mm) without the nominal-name convention. The wood is the same; only the labelling differs.
Why is the deduction ½ inch instead of ¼ inch for 2× lumber?
Thicker rough lumber loses more in surfacing because there is more wood to plane down to flat parallel faces. PS 20 allows ½ inch off both thickness and width for nominal 2-inch to 4-inch lumber, and ¾ inch off both for nominal sizes above 4 inches. Boards 1 inch thick and under lose only ¼ inch off each dimension because the thinner stock can be surfaced more closely.
How do I figure stud-wall thickness?
A 2×4 wall is 3½ inches thick (the actual width of the stud). Add drywall on both faces — typically ½ inch each side — and the finished wall is 4½ inches. A 2×6 wall is 5½ inches thick studs, 6½ inches finished. Insulated 2×8 walls (for advanced framing or high-R assemblies) are 7¼ inches thick studs, 8¼ inches finished.
Where do these standards live?
United States: Voluntary Product Standard PS 20, American Softwood Lumber Standard, published by the National Institute of Standards and Technology. Canada: CSA O141, Softwood Lumber. The actual dimensions are also reproduced in every NDS (National Design Specification for Wood Construction) and every building code that references span tables for lumber, so they are effectively universal across both countries.