Nominal vs Actual Lumber Sizes
Everything you need to know about why a 2x4 is 1.5 by 3.5 and how to avoid costly mistakes.
What Is the Difference?
The nominal size is what the lumber is called. The actual size is what it measures after it has been dried and planed smooth. When a tree is first cut into rough lumber at the sawmill, a 2×4 starts as a rough piece about 2 inches by 4 inches. Then it goes through a kiln to dry it, which causes shrinkage, and then through a planer to make the surfaces smooth and consistent. By the time it reaches the lumber rack, it is 1.5 inches by 3.5 inches.
This is standard across the industry and has been for decades. It applies to all dimensional lumber sold in North America.
Full Reference Table
| Nominal | Actual (inches) | Actual (mm) | Common Uses |
|---|---|---|---|
| 1×2 | 0.75 × 1.5 | 19 × 38 | Furring strips, small trim |
| 1×3 | 0.75 × 2.5 | 19 × 64 | Battens, small cleats |
| 1×4 | 0.75 × 3.5 | 19 × 89 | Shelving, trim, bracing |
| 1×6 | 0.75 × 5.5 | 19 × 140 | Furniture, shelving |
| 1×8 | 0.75 × 7.25 | 19 × 184 | Shelving, bench tops |
| 1×10 | 0.75 × 9.25 | 19 × 235 | Wide shelving |
| 1×12 | 0.75 × 11.25 | 19 × 286 | Wide shelves, benchtops |
| 2×2 | 1.5 × 1.5 | 38 × 38 | Stakes, small supports |
| 2×3 | 1.5 × 2.5 | 38 × 64 | Framing, blocking |
| 2×4 | 1.5 × 3.5 | 38 × 89 | Wall framing, general |
| 2×6 | 1.5 × 5.5 | 38 × 140 | Deck joists, headers |
| 2×8 | 1.5 × 7.25 | 38 × 184 | Floor joists, beams |
| 2×10 | 1.5 × 9.25 | 38 × 235 | Floor joists, rafters |
| 2×12 | 1.5 × 11.25 | 38 × 286 | Headers, stair stringers |
| 4×4 | 3.5 × 3.5 | 89 × 89 | Deck posts, columns |
| 4×6 | 3.5 × 5.5 | 89 × 140 | Beams, heavy framing |
| 6×6 | 5.5 × 5.5 | 140 × 140 | Heavy posts, beams |
Does This Apply to Hardwood Too?
Hardwood is sold differently. When you buy 4/4 (four-quarter) red oak, it is rough-sawn and measures about 1 inch thick. After planing, it might be 7/8 or 13/16 depending on how much you surface. The width is whatever the board happens to be, and you pay for the actual board feet based on the rough dimensions.
Hardwood suppliers typically list thickness in quarters: 4/4 (1"), 5/4 (1.25"), 6/4 (1.5"), 8/4 (2"), etc. These are nominal rough thicknesses. The actual thickness after milling depends on how smooth you want the surface. This is different from construction lumber where the nominal and actual sizes are standardized.
Why This Matters for Your Calculator
If you are entering dimensions into a board foot calculator and you use nominal sizes (2 × 4 instead of 1.5 × 3.5), your result will be off by roughly 25-30%. For a project using 100 board feet, that is a significant miscalculation. This calculator uses actual dimensions in its presets so you do not have to worry about it, but if you are entering custom sizes, make sure you are using the actual measurements.
Use actual dimensions: The board foot calculator on this site has Quick Fill presets with actual dimensions for common lumber sizes, so your calculations are always accurate.