Nominal vs. actual dimensions
Board footage is priced on nominal size, but the wood you receive is milled to a smaller actual (dressed) size — the difference matters for anything measured to a tight tolerance.
| Nominal size | Actual (dressed) size | Typical use |
|---|---|---|
| 1×4 | 3/4 in × 3-1/2 in | Trim, shelving |
| 2×4 | 1-1/2 in × 3-1/2 in | Wall framing studs |
| 2×6 | 1-1/2 in × 5-1/2 in | Joists, framing |
| 2×10 | 1-1/2 in × 9-1/4 in | Floor/roof joists, stringers |
| 2×12 | 1-1/2 in × 11-1/4 in | Beams, headers, stair stringers |
- Board-foot pricing uses nominal (not actual dressed) dimensions by long-standing lumber industry convention, regardless of species or surfacing.
- Hardwood suppliers often price by board foot even for surfaced (S2S/S4S) stock; ask whether the quoted price is nominal or a specific measured (actual) board footage for rough-sawn hardwood.
What is a board foot?
A board foot is a unit of lumber volume equal to a piece of wood 1 inch thick, 12 inches wide and 12 inches long — 144 cubic inches. It is the standard unit US hardwood dealers and many softwood lumber yards use for pricing, because it lets boards of very different shapes (thick and narrow, thin and wide) be compared and priced on the same basis.
Board footage is conventionally calculated from a board's nominal dimensions — the size it is called by (e.g., a '2×10') — rather than its smaller actual dressed (surfaced) size. This is a pricing and ordering convention set by lumber grading rules, not a claim that the finished board measures exactly 2 inches by 10 inches.
How to use this board foot calculator
- Enter the board's nominal thickness in inches (e.g., 2 for a 2× board).
- Enter the board's nominal width in inches (e.g., 10 for a 2×10).
- Enter the board's length in feet.
- Enter how many identical pieces you need, and optionally a price per board foot to get a total cost.
- Read the total board feet, the board feet per single piece, and the estimated cost if a price was entered.
The formula behind board feet
Board feet per piece equal nominal thickness (inches) multiplied by nominal width (inches) multiplied by length (feet), divided by 12. Dividing by 12 converts the inch-inch-foot product into the 144-cubic-inch board-foot unit. Total board feet is this per-piece figure multiplied by the number of pieces.
Worked example: a nominal 2×10 board, 12 feet long, gives (2 × 10 × 12) ÷ 12 = 20 board feet for one piece. Ordering 8 identical pieces gives 20 × 8 = 160 total board feet — the figure a lumber yard would use to price the order.
Common mistakes
- Using actual dressed dimensions (e.g., 1.5 in × 9.25 in) instead of the nominal size (2 in × 10 in) when the yard prices by nominal board footage — this understates the board feet and the price.
- Entering length in inches instead of feet, which throws the result off by a factor of 12.
- Forgetting the quantity multiplier and pricing only a single piece instead of the full order.
- Assuming board-foot pricing conventions are identical everywhere — some hardwood dealers measure and price actual rough-sawn footage per board rather than nominal size, so confirm the supplier's method before ordering.
常见问题
What is a board foot in lumber?
A board foot is a volume unit equal to 144 cubic inches — a piece of wood 1 inch thick, 12 inches wide and 12 inches long — used by US lumber yards and hardwood dealers to price and compare boards of different sizes on a common basis.
How do you calculate board feet?
Multiply the nominal thickness in inches by the nominal width in inches by the length in feet, then divide by 12. For example, a 2×10 board 12 feet long is (2 × 10 × 12) ÷ 12 = 20 board feet.
Is board footage based on nominal or actual lumber size?
By long-standing industry convention, board footage is calculated from a board's nominal size (the size it is called, e.g., 2×10) rather than its smaller actual dressed size, even though the physical board measures less after milling.
How many board feet are in a 2×4×8?
Using nominal dimensions: (2 × 4 × 8) ÷ 12 ≈ 5.33 board feet for one 2×4 that is 8 feet long.
Why do lumber yards price by board foot instead of by piece?
Board-foot pricing lets a yard set one price per unit of wood volume that applies fairly across boards of very different thickness, width and length, rather than needing a separate price list for every possible dimension combination.
参考文献
- National Hardwood Lumber Association (NHLA) — Rules for the Measurement & Inspection of Hardwood & Cypress: defines board-foot measurement conventions for hardwood.
- American Softwood Lumber Standard, PS 20 (US Dept. of Commerce / NIST): defines nominal vs. actual dressed dimensions for softwood lumber.
- USDA Forest Products Laboratory — Wood Handbook: Wood as an Engineering Material: reference for lumber volume and measurement conventions.