WORKBENCH GUIDE
Lumber sizes and grades: nominal vs actual
A 2 × 4 is not 2 by 4 inches. How nominal sizes, dry and green dressed sizes and hardwood grades work, from the Wood Handbook.
Nominal and actual sizes
Lumber length is recorded in actual dimensions, but width and thickness are traditionally given as nominal sizes, larger than the actual ones. The American Softwood Lumber Standard PS 20 sets the minimum dressed sizes: a nominal 2 × 4 is 1-1/2 × 3-1/2 in (38 × 89 mm) when dry.
Because lumber can be surfaced green or dry, the standard gives both sizes: green lumber is surfaced slightly larger so that it shrinks to about the dry size as it dries to about 15% moisture content. Dry lumber, in this standard, has a maximum moisture content of 19%.
| Nominal | Dry | Green |
|---|---|---|
| 1 in | 3/4 in (19 mm) | 25/32 in (20 mm) |
| 1-1/4 in | 1 in (25 mm) | 1-1/32 in (26 mm) |
| 1-1/2 in | 1-1/4 in (32 mm) | 1-9/32 in (33 mm) |
| 2 in | 1-1/2 in (38 mm) | 1-9/16 in (40 mm) |
| 3 in | 2-1/2 in (64 mm) | 2-9/16 in (65 mm) |
| 4 in | 3-1/2 in (89 mm) | 3-9/16 in (90 mm) |
| 6 in | 5-1/2 in (140 mm) | 5-5/8 in (143 mm) |
| 8 in | 7-1/4 in (184 mm) | 7-1/2 in (190 mm) |
| 10 in | 9-1/4 in (235 mm) | 9-1/2 in (241 mm) |
| 12 in | 11-1/4 in (286 mm) | 11-1/2 in (292 mm) |
Softwood grades
Dimension lumber, nominal 2 to 4 in thick, is stress graded under the National Grading Rule: Select Structural, No. 1, No. 2 and No. 3, and for 2 × 4s also Construction, Standard, Utility and Stud. Boards, less than nominal 2 in thick, are graded for appearance and use; retail yards commonly stock No. 1, No. 2 and No. 3 Common.
Kiln-dried softwood dimension lumber generally averages 19% moisture content or less, and lumber grade-marked KD 15 is generally limited to 15%. Furniture and cabinet lumber is usually dried to 6–8%, so construction lumber needs time to acclimatise before fine work.
Hardwood grades
Hardwood lumber is graded under the National Hardwood Lumber Association rules by the clear cuttings it can yield. The grades run FAS (Firsts and Seconds), F1F (FAS one face), Selects, No. 1 Common, No. 2A and 2B Common, No. 3A and 3B Common and Sound Wormy. FAS must allow at least 83-1/3% of its surface measure to be cut into clear-face cuttings, No. 1 Common 66-2/3%.
Minimum widths rise with the grade: 6 in for FAS and F1F, 4 in for Selects and 3 in for the Common grades. The rules are written only in inch–pound units, and except for F1F and Selects the poorer face of the board is the one inspected.
Buying by volume
Board feet and cubic metres depend on the agreed basis, nominal or actual sizes. Ask which basis the supplier uses and record it with the quantity, as the board-feet calculator does.
Ten 2 × 6 boards, 10 ft long: nominal 10 × 2 × 6 × 10 / 12 = 100 board feet; at the actual dry size of 1-1/2 × 5-1/2 in the same boards hold 68.75 board feet of wood.