Nominal and actual dimensions for softwood dimensional lumber, in inches and millimetres, with the subtractive rule that generates them.
Data verified 2026-09-29 · based on PS 20-20
| Nominal[2] | Actual thickness in[2] | Actual width in[2] | Actual thickness mm[1] | Actual width mm[1] | Typical use[2] |
|---|---|---|---|---|---|
| 1×2 # | 0.75 | 1.50 | 19.0 | 38.1 | Furring strips, lattice, light trim |
| 1×3 # | 0.75 | 2.50 | 19.0 | 63.5 | Furring, trim, light framing |
| 1×4 # | 0.75 | 3.50 | 19.0 | 88.9 | Trim, shelving, light framing |
| 1×6 # | 0.75 | 5.50 | 19.0 | 139.7 | Fence boards, shelving, trim |
| 1×8 # | 0.75 | 7.25 | 19.0 | 184.1 | Shelving, siding, fascia |
| 1×10 # | 0.75 | 9.25 | 19.0 | 234.9 | Shelving, fascia, stair treads (with support) |
| 1×12 # | 0.75 | 11.25 | 19.0 | 285.8 | Shelving, siding, stair risers |
| 2×2 # | 1.50 | 1.50 | 38.1 | 38.1 | Light framing, blocking, spindles |
| 2×3 # | 1.50 | 2.50 | 38.1 | 63.5 | Light framing, blocking, non-load-bearing partitions |
| 2×4 # | 1.50 | 3.50 | 38.1 | 88.9 | The general-purpose framing member — studs, plates, rafters, joists |
| 2×6 # | 1.50 | 5.50 | 38.1 | 139.7 | Floor joists, rafters, deck framing |
| 2×8 # | 1.50 | 7.25 | 38.1 | 184.1 | Floor joists, rafters, headers |
| 2×10 # | 1.50 | 9.25 | 38.1 | 234.9 | Floor joists, rafters, large headers |
| 2×12 # | 1.50 | 11.25 | 38.1 | 285.8 | Long-span floor joists and rafters |
| 4×4 # | 3.50 | 3.50 | 88.9 | 88.9 | Posts, deck columns, blocking |
| 4×6 # | 3.50 | 5.50 | 88.9 | 139.7 | Beams, posts, heavy blocking |
| 6×6 # | 5.50 | 5.50 | 139.7 | 139.7 | Structural posts, heavy beams, retaining walls |
| 8×8 # | 7.25 | 7.25 | 184.1 | 184.1 | Heavy structural posts and timber framing |
Actual dimensions are for dry, surfaced softwood at 19% moisture content or less, which is how lumber is sold at retail. Green or unseasoned lumber is cut larger to allow for shrinkage and measures differently. Hardwood is sold by the quarter — 4/4, 5/4, 8/4 — where the nominal thickness is in quarter-inches before surfacing, so 4/4 rough-surfaced-one-side is about 13/16 in thick. Nominal sizes are also not measurements of anything the buyer receives; they identify the product, which is why a 2×4 is still called a 2×4 after a century of shrinking actual dimensions.
Lumber is sawn oversize and then planed smooth on all four faces. The planing removes material from both faces and both edges, so the finished piece is smaller than the nominal rough size in every dimension — and the amount removed is set by the standard rather than by the mill.
Under the American Softwood Lumber Standard, the reduction below 2 inches nominal is 1/4 in for thicknesses under 1 in, 1/2 in for thicknesses of 1 in and above, and 1/2 in from the width up to 8 in. That single rule generates almost every row in the table: a 2×4 becomes 1-1/2 × 3-1/2, a 1×6 becomes 3/4 × 5-1/2, a 2×8 becomes 1-1/2 × 7-1/4.
Above 8 in the width reduction increases to 3/4 in, which is why a 2×10 is 9-1/4 in wide rather than 9-1/2. Above 2 in nominal thickness the thickness reduction also increases to 3/4 in, which is why a 6×6 measures 5-1/2 × 5-1/2. Both exceptions are worth remembering, because the simple subtract-a-half rule gives the wrong answer at exactly the sizes where a beam calculation is most likely to matter.
The nominal size survives because it is a product identifier rather than a measurement. A 2×4 has been called a 2×4 since long before the current planing standards, and changing the name would break every reference in every code book and catalogue.
Structural calculations always use actual dimensions. The section properties tabulated in span tables and engineering references — area, section modulus, moment of inertia — are computed from the actual size, and substituting the nominal size overstates them badly. A 2×4's actual cross-section is 5.25 in² against the 8 in² the nominal name implies, a 34% overstatement.
The same applies to depth, where the error is worse because bending stiffness scales with the cube of depth and strength with the square. A 2×10 is 9.25 in deep, not 10 in: the actual stiffness is about 79% of what the nominal depth would give, and the actual bending strength about 86%. Sizing a joist from nominal dimensions is not a small error.
Two further practical points. Nominal sizes are stable across the industry, but actual dimensions vary slightly with moisture content and with the surfacing pattern — surfaced-four-sides, surfaced-one-side and rough all differ. And engineered lumber — LVL, glulam, I-joists, LSL — is sold in actual dimensions, with no nominal convention at all, so a 1-3/4 × 9-1/2 LVL really is 1-3/4 × 9-1/2 inches. Mixing nominal-dimension lumber with actual-dimension engineered lumber on the same plan is a common source of framing errors.
Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.
| # | Source | Type | Revision / method |
|---|---|---|---|
| [1] | Metric dimensions converted from the standard inch dimensions | derived | Actual inch dimensions converted at exactly 25.4 mm per inch, then rounded to the nearest millimetre. The metric lumber trade uses its own rounded sizes — 38 × 89 mm rather than 38.1 × 88.9 mm — so the metric column is a conversion of the imperial standard, not the metric standard. |
| [2] | PS 20 — American Softwood Lumber Standard, and NIST Handbook 130 for the retail dimension rules | standard | PS 20-20 |
| Standard | Revision | What it covers on this page |
|---|---|---|
| PS 20 — American Softwood Lumber Standard | PS 20-20 | the nominal sizes and their standard surfaced dimensions |
| NIST Handbook 130 — Uniform Laws and Regulations, Method of Sale of Commodities | NIST Handbook 130, 2024 edition | the retail dimension requirements for lumber sold by nominal size |
| ASTM D9 — Standard Terminology Relating to Wood and Wood-Based Products | ASTM D9-23 | the definitions of nominal, rough and surfaced |
Cross-checked against:
Derived values — the following values on this page are calculated, not taken directly from the standard:
| Value | How it is derived |
|---|---|
| Actual thickness and width in millimetres | Inch dimension × 25.4, rounded to the nearest millimetre and expressed to one decimal place. Checked at build time against every row to within 0.5%. |
Actual dimensions apply to dry, surfaced softwood at 19% moisture content or less. Green lumber, rough-sawn lumber, hardwood sold by the quarter and engineered wood products all measure differently, and engineered lumber uses actual dimensions with no nominal convention. For structural work, use the actual dimensions published in the span table you are designing from rather than converting from nominal.
Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:
Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.
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