Why a 2 × 4 is not 2 by 4 inches
A softwood board’s name — 1 × 6, 2 × 4 — is its nominal size, roughly the size it was sawn. Drying and planing take it down to the dressed size, the actual dimension you measure. The American Softwood Lumber Standard (PS 20) sets the minimum dressed sizes, and the chart above gives the dry sizes: a 2 × 4 is 1 1/2 × 3 1/2 in, a 1 × 6 is 3/4 × 5 1/2 in.
- Dry means 19% moisture content or less. Green lumber is dressed larger so that it shrinks to about the same size; those sizes are not shown here.
- Sizes move with moisture. NIST notes about 1% change in size for every 4% change in moisture content, so a board can measure slightly over or under after it has stood in the shop.
- Minimums, not targets. The dressed sizes are minimums for the grade; a board may measure a hair larger.
Hardwood: quarters, not nominal sizes
Hardwood is not sold by PS 20 nominal sizes. It is named by its rough thickness in quarter inches — 4/4 is 1 in, 8/4 is 2 in — and usually comes in random widths and lengths. Surfaced two sides (S2S), the handbook’s standard thicknesses are 3/16 in under the rough size up to 6/4 and 1/4 in under from 8/4 up: 4/4 surfaces to 13/16 in, 8/4 to 1 3/4 in.
Workshop tip— Buying for a finished thickness
For parts finished at 3/4 in, buy 4/4 and flatten it yourself. For 1 in finished, 5/4 leaves room for flattening a cupped or twisted board; 4/4 rarely does.
Worked example
A frame needs rails 3 in wide finished from 3/4 in stock. In softwood, a 1 × 4 (3/4 × 3 1/2 in actual) gives the thickness as bought and leaves 1/2 in to rip off. In hardwood, buy 4/4 and plane it from 1 in rough to 3/4 in — then count board feet on the rough 1 in thickness, as hardwood is usually tallied.
To turn sizes into an order, use the board foot calculator. For how hard a species is to dent and work, see the Janka hardness scale; surfaced boards still need sanding before finish, and the sandpaper grit chart gives the sequence.
Sources
The PS 20 sizes are the American Softwood Lumber Standard’s own, as tabulated by NIST; the standard itself was not reviewed directly. Hardwood surfaced thicknesses are the handbook’s standard values; mills and yards may surface to other thicknesses by agreement.
- 1.
NIST Handbook 130 (2026), IV.B Uniform Regulation for the Method of Sale of Commodities. National Institute of Standards and Technology, 2026.
Used for: Softwood nominal and dry minimum dressed sizes under the American Softwood Lumber Standard, PS 20-20 (section 2.10, Table 1); dry defined as 19% moisture content or less; the note that size changes about 1% for each 4% change in moisture content.
- 2.
Christopher Adam Senalik and Benjamin Farber. Chapter 6: Commercial Lumber, Round Timbers, and Ties. USDA Forest Service, Forest Products Laboratory — Wood Handbook, FPL-GTR-282, 2021.
Used for: Standard hardwood thicknesses, rough and surfaced two sides (Table 6–4), 4/4 to 16/4.
About this chart
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