Sources and scope for this page: PS 20-20, covering the nominal sizes and their standard surfaced dimensions; and NIST Handbook 130, 2024 edition, covering the retail dimension requirements for lumber sold by nominal size (one more source is tabulated below). No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. Full sources, values and limits on this page.
Page last changed 2026-11-08 · based on PS 20-20
Quick Answer
Nominal and Actual Lumber Dimensions
| 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.
Where the Reduction Comes From
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.
Designing With Actual Dimensions
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.
Frequently Asked Questions
- Why is a 2x4 not 2 inches by 4 inches?
- Because the nominal size refers to the rough-sawn dimensions before planing, and a 2×4 is planed smooth on all four faces. The standard reduction — 1/2 in from the thickness and 1/2 in from the width for lumber under 2 in nominal — leaves 1-1/2 in × 3-1/2 in. The nominal name has been kept as a product identifier even though it no longer describes the piece.
- What are the actual dimensions of a 2x10?
- 1-1/2 in × 9-1/4 in, or 38 mm × 235 mm. Note that a 2×10 loses 3/4 in from its width, not the 1/2 in that applies up to 8 in nominal — the width reduction increases above 8 in. A 2×8 is 7-1/4 in wide for the same reason, and a 2×6 is 5-1/2 in.
- How do I calculate board feet?
- Board feet is a volume measure: nominal thickness in inches × nominal width in inches × length in feet, divided by 12. Note that the trade convention uses nominal dimensions, so a 2×4 eight feet long is 5.33 board feet even though its actual volume is only about 3.5 board feet. Retail sales of dimensional lumber are by the piece, not by board feet; the measure is used for hardwood and for log scaling.
- Do metric lumber sizes use different actual dimensions?
- Yes, and the difference matters. Metric-dimension softwood — common in Europe and Australia — is sold at actual metric dimensions, so a 45 × 90 mm stud really is 45 × 90 mm, with no nominal-versus-actual gap. The nearest equivalent to a North American 2×4 is 38 × 89 mm, which is close but not identical, so metric and imperial framing hardware and spacing do not always interchange.
Related
Value Sources
References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.
| # | 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 | reference | PS 20-20 |
Data Sources
| 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 |
Verification scope
No page-specific independent source review is recorded here. The listed references and formulas explain the basis of the compilation; automated consistency checks do not verify the original sources or establish product conformity.
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.
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