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Calculate board feet for lumber purchases instantly. A board foot (BF) is the standard unit for measuring hardwood lumber volume, equal to 144 cubic inches or 1/12 cubic foot.
Where T = thickness (in), W = width (in), L = length (ft)
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Unlike square footage, which measures area, board footage measures volume. You use it when purchasing multiple boards of lumber in various sizes.
By definition, one board foot of lumber is one square foot that is one-inch thick. If you would like to convert regular volume units into board feet, use the following relation:
1 board foot = 144 cubic inches = 1/12 cubic foot
All you need to do is use the board foot formula:
Pay special attention to the units! The length of the wooden board should be expressed in feet, while the width and thickness should be in inches.
5 boards: 8 ft long, 10 in wide, 1.25 in thick
BF per board = (8 × 10 × 1.25) / 12
BF per board = 100 / 12 = 8.33 BF
Total = 5 × 8.33 = 41.67 BF
At $4.15/BF: 41.67 × $4.15 = $172.93
Quick reference showing board feet for standard 2x4 lumber at various lengths:
| Length of 2x4 (ft) | Number of Board Feet |
|---|---|
| 6 | 4 |
| 8 | 5⅓ |
| 10 | 6⅔ |
| 12 | 8 |
| 14 | 9⅓ |
| 16 | 10⅔ |
| 18 | 12 |
| 20 | 13⅓ |
| 22 | 14⅔ |
| 24 | 16 |
Based on actual 2x4 dimensions: 1.5 in thick × 3.5 in wide
| Nominal Size | Actual Size | BF per Foot of Length | BF per 8 ft Board |
|---|---|---|---|
| 1x4 | 3/4 × 3-1/2 in | 0.219 BF | 1.75 BF |
| 1x6 | 3/4 × 5-1/2 in | 0.344 BF | 2.75 BF |
| 1x8 | 3/4 × 7-1/4 in | 0.453 BF | 3.63 BF |
| 1x12 | 3/4 × 11-1/4 in | 0.703 BF | 5.63 BF |
| 2x4 | 1-1/2 × 3-1/2 in | 0.438 BF | 3.50 BF |
| 2x6 | 1-1/2 × 5-1/2 in | 0.688 BF | 5.50 BF |
| 2x8 | 1-1/2 × 7-1/4 in | 0.906 BF | 7.25 BF |
| 2x10 | 1-1/2 × 9-1/4 in | 1.156 BF | 9.25 BF |
| 2x12 | 1-1/2 × 11-1/4 in | 1.406 BF | 11.25 BF |
| 4x4 | 3-1/2 × 3-1/2 in | 1.021 BF | 8.17 BF |
Hardwood is sold in quarters of an inch. For example, 4/4 (four-quarter) lumber is 1 inch thick when rough-sawn. After surfacing, you lose about 1/4 inch.
| Quarters | Rough | Surfaced (S2S) |
|---|---|---|
| 4/4 | 1 inch | 3/4 inch |
| 5/4 | 1-1/4 inch | 1 inch |
| 6/4 | 1-1/2 inch | 1-1/4 inch |
| 8/4 | 2 inches | 1-3/4 inch |
| 10/4 | 2-1/2 inches | 2-1/4 inch |
| 12/4 | 3 inches | 2-3/4 inch |
S2S = Surfaced 2 Sides (faces planed smooth)
S4S = Surfaced 4 Sides (faces and edges planed)
Rough = Not surfaced, as it came from the sawmill
Unlike dimensional lumber (2x4, 2x6), hardwood comes in random widths and lengths. Board feet allows fair pricing regardless of board dimensions.
A board foot measures volume (length × width × thickness), while a linear foot only measures length (12 inches). Linear feet ignores width and thickness completely.
Use the Doyle Rule: ((Diameter - 4)/4)² × Length, where diameter is the small end in inches (excluding bark) and length is in feet. This estimates recoverable board feet after milling.
A board foot of oak weighs 3.08-4.67 lbs (average 3.875 lbs) depending on variety. Oak density ranges from 37-56 lb/ft³.
You cannot directly convert square feet to board feet since board feet measures volume. However, 320 square feet of 1-inch thick material equals 320 board feet. Multiply by thickness in inches for other thicknesses.
In spray foam insulation, a board foot measures coverage volume (1 ft × 1 ft × 1 in). Estimate by calculating square footage and multiplying by desired thickness in inches.
Add 15-25% extra for waste from cutting, defects, and mistakes. For complex projects or premium work, add up to 30% extra.
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