Square feet = linear feet × board width in inches ÷ 12, so 100 linear feet of a 5 1/2 in wide board covers 45.83 square feet. The calculator also works backwards: 100 square feet needs 218.2 linear feet of that board.
How to convert linear feet to square feet
Square feet = linear feet × width (in) ÷ 12 Linear feet = square feet × 12 ÷ width (in)
A linear foot has length only. To get area you need the second dimension, the width of the board, strip or roll, because a 2 in trim board and a 12 in shelf board are both one linear foot per foot of length and cover very different areas. Convert the width to feet (divide by 12) and multiply: 5 1/2 in is 0.4583 ft, and 100 ft × 0.4583 ft = 45.83 sq ft.
Use the actual width, not the name of the board. A 1x6 or 2x6 is 5 1/2 in wide, a 1x8 is 7 1/4 in and a 1x4 is 3 1/2 in (NIST PS 20-25, Table 3). The chips under the width field fill those numbers in.
Square feet to linear feet
Divide instead. To cover 100 sq ft with 5 1/2 in boards you need 100 × 12 ÷ 5.5 = 218.2 linear feet. Add waste on top before you buy: 10% makes 240 linear feet, which is 30 boards of 8 ft.
Linear feet in a wall, floor or deck
For a floor or deck, use the covered width of a board, which is the face width plus any gap. A 12 ft by 16 ft deck is 192 sq ft. With 5 1/2 in boards laid tight it takes 192 × 12 ÷ 5.5 = 418.9 linear feet; with a 1/8 in gap the covered width is 5 5/8 in and it takes 409.6 linear feet. For real decking counts with joists, use the deck board calculator.
For tongue and groove flooring or siding, use the width that shows once it is installed, not the width of the whole board.
Fabric, roofing paper and rolls
Rolls work the same way. A 4 ft wide (48 in) roll that is 100 linear feet long covers 400 sq ft, and 1,000 sq ft of area needs 250 linear feet of it before overlaps.
Square feet per 100 linear feet by board width
| Board width | Sq ft per 100 linear feet | Linear feet per 100 sq ft |
|---|---|---|
| 1 1/2 in | 12.5 | 800 |
| 2 1/2 in | 20.8 | 480 |
| 3 1/2 in | 29.2 | 343 |
| 5 1/2 in | 45.8 | 218 |
| 7 1/4 in | 60.4 | 166 |
| 9 1/4 in | 77.1 | 130 |
| 11 1/4 in | 93.8 | 107 |
The widths are the dressed sizes of softwood lumber in NIST Voluntary Product Standard PS 20-25.
When linear feet are not enough
Boards are also sold by volume. Board feet multiplies in the thickness, so use the board foot calculator for rough lumber and the lumber calculator for a purchase list. To add up many lengths first, use the linear feet calculator, and to measure the area of a room instead, use the square footage calculator. The feet and inches calculator adds tape readings. All the conversion tools are listed in measuring and planning calculators.