Lumberyards, flooring suppliers and sheet-goods counters quote in three different feet, and they are not interchangeable. One measures length, one area and one volume. Once you see which dimension each unit ignores, converting between them stops being guesswork: you supply the missing dimension and multiply.
Three Units, One Dimension Apart
A linear foot ignores everything except length. A 1x2 strip and a 6x6 post are both sold in linear feet if the price is per foot of length, and twelve linear feet of either is twelve feet end to end.
A square foot adds width. It is the area of a 12 × 12 in square, 144 in², whatever the thickness. Floors, decks, walls and sheets are measured this way because the job is to cover a surface.
A board foot adds thickness. It is the volume of a board 1 in thick, 12 in wide and 12 in long, 144 in³. Rough hardwood, slabs and stock in mixed sizes are measured this way because only volume compares them fairly.
| Unit | Measures | Equal to | To convert you also need |
|---|---|---|---|
| Linear foot (LF) | Length | 12 in, or 0.3048 m | Width to get area; width and thickness to get volume |
| Square foot (ft²) | Area | 144 in², or 0.0929 m² | Thickness to get volume; face width to get length |
| Board foot (BF) | Volume | 144 in³, or 0.00236 m³ | Thickness and width to get length |
Which Unit Goes With Which Material
The unit follows what varies. When the cross-section is fixed, as with moulding, a length is enough. When the job is to cover a surface, area is the natural measure. When thickness and width vary from board to board, only volume compares them.
These are common patterns, not rules. The same deck board can be quoted per piece, per linear foot or per square foot of deck, so always check the basis of a quote before comparing it with another.
- Rough hardwood lumber and slabs: board feet, because widths and lengths are random.
- Softwood dimension lumber: by the piece or per linear foot at the counter, tallied in board feet on nominal sizes in the trade.
- Trim, moulding, skirting and handrail: linear feet of a fixed profile.
- Decking and cladding: linear feet or pieces, converted from the square feet to be covered.
- Plywood, MDF and OSB: by the sheet, with square feet used to work out how many sheets an area needs.
Conversions You Will Actually Use
Every conversion below either multiplies by the missing dimension or divides by it. Keep thickness and width in inches and lengths and areas in feet, and the factor of 12 takes care of the rest.
For board feet and linear feet of a given size, the board foot to linear foot calculator does the arithmetic both ways and shows the nominal and dressed results side by side.
Board feet = linear feet × thickness (in) × width (in) / 12
Linear feet = board feet × 12 / (thickness × width)
Board feet = square feet × thickness (in)
Linear feet to cover an area = square feet × 12 / (face width + gap, in)
Square feet = linear feet × face width (in) / 12
Worked Example: Pricing Hardwood by Volume and by Length
A table needs 30 linear feet of rough walnut 1 in thick and 6 in wide. Its volume is 1 × 6 × 30 / 12 = 15 board feet.
At an illustrative $12.00 per board foot that is $180.00. The same wood priced per linear foot would have to be $6.00 per foot to match, because each foot of 1 × 6 in stock is half a board foot. Double the thickness and the per-foot price doubles, while the per-board-foot price stays put.
To total mixed sizes and add a price, use the board foot calculator.
The lumber cost calculator adds a waste allowance and compares supplier quotes on one basis.
Worked Example: Square Feet of Deck to Linear Feet of Board
A 12 × 10 ft deck is 120 ft². Boards with a 5-1/2 in face laid with an illustrative 1/8 in gap cover 5.625 in per row, so the deck needs 120 × 12 / 5.625 = 256 linear feet of board before any allowance for waste and offcuts.
The nominal width is not the face you lay. A nominal 6 in deck board is narrower once dressed, and using 6 in in the formula would order too little. The table below gives the linear feet per square foot for common board faces, laid edge to edge with no gap.
For row counts, gaps and stock lengths, the deck board calculator works from the deck size directly.
| Board | LF per ft² covered | BF per LF (nominal) | BF per ft² covered (nominal) |
|---|---|---|---|
| 1x4 (3.5 in face) | 3.429 | 0.333 | 1.143 |
| 1x6 (5.5 in face) | 2.182 | 0.500 | 1.091 |
| 1x8 (7.25 in face) | 1.655 | 0.667 | 1.103 |
| 1x10 (9.25 in face) | 1.297 | 0.833 | 1.081 |
Worked Example: A Sheet in Square Feet and Board Feet
A 4 × 8 ft sheet of 3/4 in plywood covers 32 ft². Multiplied by its 0.75 in thickness it holds the volume of 24 board feet. Sheets are not sold by the board foot, but the conversion is useful when comparing a panel with solid wood for the same part, or estimating weight from a density per board foot.
To work out how many sheets a cut list really needs, which depends on how the parts fit rather than on area alone, see the sheet goods calculator.
Nominal or Actual: Which Size the Feet Are Counted On
For North American softwood, the American Softwood Lumber Standard defines board measure on the nominal thickness and width. A 2x4 is tallied as 2 × 4 in, giving 0.667 board feet per foot, although the dressed board is 1-1/2 × 3-1/2 in and holds only 0.4375 board feet of wood per foot. The standard uses dressed sizes for cubic measure and warns against mixing the two.
Square feet of coverage work the other way: the face that is laid is the dressed width, so use the actual size. Mixing the two bases in one estimate is the most common source of a short or long order.
The difference between labels and real boards is covered in Nominal vs Actual Lumber Sizes.
Common Mistakes
Each of these comes from dropping or double-counting one of the three dimensions.
- Converting square feet of deck to board feet instead of linear feet.
- Using nominal width to cover an area, which orders too little.
- Comparing a price per linear foot with a price per board foot without converting.
- Forgetting that a square foot has no thickness, so square feet alone never give a volume.
- Adding waste to the square feet and again to the linear feet.
Common questions
Is a linear foot the same as a board foot?
Only for a board with 12 in² of cross-section, such as a nominal 2x6 or 1x12. For a 1x6 one board foot is 2 linear feet, and for a 2x12 it is half a linear foot.
How do I convert square feet to board feet?
Multiply the square feet by the thickness in inches. One square foot of 1 in stock is one board foot; one square foot of 3/4 in stock is 0.75 board feet.
How many linear feet of boards do I need for 100 square feet?
Divide 100 × 12 by the face width plus gap in inches. With 5-1/2 in faces and no gap that is about 218 linear feet, before waste.
Why do lumberyards use board feet if the boards are smaller?
Board measure is the trade tally that the American Softwood Lumber Standard defines on nominal sizes. The wood in the board is smaller, which matters for volume and weight but not for the tally.
Sources
- Voluntary Product Standard PS 20-20: American Softwood Lumber Standard (Revision 1, October 2021) · U.S. Department of Commerce, NIST
Section 2.2, board measure: board feet are the nominal thickness in inches times the nominal width in feet times the length in feet. Section 4.1: lumber is tallied by board measure or cubic measure, and dressed sizes are used for cubic measure. Table 3 gives the minimum dressed dry sizes used for the dressed option, and Appendix B warns that nominal board feet and dressed cubic metres are not the same volume.
- Valuing Standing Timber · Penn State Extension
Defines the board foot as 144 cubic inches, a piece 1 in thick, 12 in wide and 12 in long.
- NIST Guide to the SI, Appendix B.9: Factors for units listed by kind of quantity · National Institute of Standards and Technology
1 ft = 0.3048 m exactly and 1 ft² = 0.09290304 m², so 1 in = 0.0254 m and 1 board foot = 144 × 0.0254³ = 0.002359737 m³.