A nominal size is a product designation. Actual size is what you can measure. Surfacing removes wood and drying can reduce dimensions, so a finished board may be smaller than the name used to sell it.
Common US dimension-lumber examples
| Nominal | Common surfaced size |
|---|---|
| 1 × 4 | ¾ × 3½ in |
| 2 × 4 | 1½ × 3½ in |
| 2 × 6 | 1½ × 5½ in |
| 4 × 4 | 3½ × 3½ in |
These are familiar examples, not guarantees for every product. Rough-sawn timber, specialty products, moisture condition, and regional practices differ. For close-fitting work, measure the stock itself.
The volume changes with the convention
An 8 ft nominal 2×4 gives 2 × 4 × 8 ÷ 12 = 5.333 board feet. Using actual 1.5 × 3.5 inch dimensions gives 3.5 board feet. The first is a nominal tally; the second describes physical volume. Each answers a different question.
The board foot calculator uses exactly the dimensions you enter. Use the supplier’s stated convention to check a quote, and actual dimensions for mass or physical fit.
Check sheet thickness as well
An 18 mm panel and a ¾ inch panel are not the same thickness. Three-quarters of an inch is exactly 19.05 mm. The 1.05 mm difference matters in a groove, rabbet, or stack of parts.
Measure before cutting joinery. For coverage, use actual exposed face width. For inset doors or drawers, measure the clear opening. Small differences accumulate across repeated parts.