A board foot is the standard unit for buying hardwood lumber. One board foot equals 144 cubic inches — a piece 12 inches long, 12 inches wide, and 1 inch thick. If you're buying walnut, cherry, maple, or any hardwood from a lumber dealer, you're paying by the board foot.
Enter your dimensions and optional price per board foot.
The board feet formula
There are two versions depending on whether your length is in inches or feet:
- Length in inches: Board Feet = (Length × Width × Thickness) ÷ 144
- Length in feet: Board Feet = (Length × Width × Thickness) ÷ 12
Width and thickness are always in inches, regardless of which formula you use.
Examples
Example 1: A board that's 8 feet long, 6 inches wide, and 1 inch thick:
Board feet = (8 × 6 × 1) ÷ 12 = 4 board feet
Example 2: A board that's 48 inches long, 8 inches wide, and 2 inches thick:
Board feet = (48 × 8 × 2) ÷ 144 = 5.33 board feet
Example 3: Estimating cost. You need 20 board feet of walnut at $12.50/bf:
Cost = 20 × $12.50 = $250.00
Nominal vs. actual thickness
Lumber dealers sell hardwood in "quarter" thicknesses:
- 4/4 (four-quarter) — nominally 1 inch. Rough-sawn is ~1", surfaced (S2S) is ~13/16".
- 5/4 — nominally 1.25 inches. Surfaced is ~1-1/16".
- 6/4 — nominally 1.5 inches. Surfaced is ~1-5/16".
- 8/4 — nominally 2 inches. Surfaced is ~1-3/4".
When buying lumber, you're charged for the nominal (rough) thickness, not the surfaced thickness. A 4/4 board is priced as 1 inch thick even if it's been planed to 13/16". Ask your dealer whether prices are for rough or surfaced stock.
How to estimate for a project
- List every part with final dimensions (length, width, thickness).
- Add waste factor. Multiply total board footage by 1.15 to 1.25 (15-25% extra) for cutting waste, defects, and grain matching.
- Round up. Lumber comes in random widths and lengths. You'll always use more than the theoretical minimum.
- Check prices. Call your lumber dealer or check online. Prices vary significantly by species, grade, and region.
For a dining table top using 8/4 walnut:
- Top: 72" × 36" × 2" = (72 × 36 × 2) ÷ 144 = 36 board feet
- With 20% waste: 36 × 1.2 = ~43 board feet
- At $14/bf: ~$602
Board feet vs. linear feet
Board feet measure volume (3 dimensions). Linear feet measure length only (1 dimension). Softwood lumber from big-box stores (2×4, 2×6, etc.) is sold by the linear foot. Hardwood from lumber dealers is sold by the board foot.
If a recipe or plan calls for "20 board feet of cherry," that's a volume — you could satisfy it with one wide board or several narrow ones, as long as the total volume adds up.
Common mistakes
- Forgetting to add waste. Theoretical board footage is always less than what you need. Knots, splits, sapwood, and grain direction all eat into usable material.
- Confusing nominal and actual thickness. You pay for nominal. You build with actual. Know the difference.
- Not accounting for surfacing. If you buy rough lumber and have it planed, you lose ~3/16" from each face. Plan your thickness accordingly.
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