How to calculate board feet

What a board foot is

A board foot is a volume: 12 inches by 12 inches by 1 inch, 144 cubic inches, one twelfth of a cubic foot. Three state extension services define it the same way and give the same formula. It is thickness in inches times width in inches times length in feet, divided by 12. If every dimension is in inches, divide by 144 instead. A 2 by 8 inch plank 24 ft long is 2 × 8 × 24 ÷ 12 = 32 board feet. That is the example Oregon State's bulletin uses. Hardwood dealers, sawmills and anyone selling rough lumber price by the board foot. Home centers price framing lumber by the piece, so the unit mostly matters once you leave the big-box aisle.

The nominal-size rule

The formula uses nominal sizes. A 2x6 is counted as 2 inches by 6 inches even though it measures 1 1/2 by 5 1/2 after drying and planing. Those are the dressed sizes in the American Softwood Lumber Standard. The Forest Products Laboratory's glossary states the convention. Board feet are figured on nominal thickness and width and actual length. Anything thinner than 1 inch is counted as 1 inch. Wisconsin's fact sheet shows the gap: a 10 ft 2x6 is 10 board feet by the trade's count but only 6.875 by its true volume. Buy by the nominal count, because that is what the invoice uses, and do not be surprised that the wood is smaller than the number.

Piece	Board feet	Per linear foot
1x4, 8 ft	2.67	0.33
1x6, 10 ft	5	0.5
1x12, 10 ft	10	1
2x4, 8 ft	5.33	0.67
2x6, 10 ft	10	1
2x8, 10 ft	13.33	1.33
2x12, 16 ft	32	2
4x4, 10 ft	13.33	1.33

Hardwood thickness in quarters

Hardwood is sold rough-sawn with its thickness named in quarters of an inch. In that system, 4/4 is one inch, 5/4 is 1 1/4, 6/4 is 1 1/2, and 8/4 is 2 inches, up through 12/4 and 16/4. The quarter figure is the rough thickness and it is the number that goes in the formula, even when the board has been surfaced thinner. The hardwood export council's statement of the grading rules gives the surfacing allowance. Subtract 3/16 inch for stock 1 1/2 inches and thinner, and 1/4 inch for 1 3/4 inches and thicker.

Quarter size	Rough thickness	Surfaced two sides	Board feet of a 6 in × 8 ft board
4/4	1 in	13/16 in	4
5/4	1 1/4 in	1 1/16 in	5
6/4	1 1/2 in	1 5/16 in	6
8/4	2 in	1 3/4 in	8
10/4	2 1/2 in	2 1/4 in	10
12/4	3 in	2 3/4 in	12

So a 5/4 board 8 inches wide and 10 ft long is 1.25 × 8 × 10 ÷ 12 = 8.33 board feet. An 8/4 board 10 inches wide and 12 ft long is 20. Counting a 5/4 board as 1 inch, a common slip, understates the bill by a fifth.

Surface measure, the dealer's shortcut

Hardwood graders work in surface measure first, the face area of the board in square feet. It is width in inches times length in feet divided by 12, rounded to the nearest whole number. A half rounds up one time and down the next, by Purdue's bulletin. Board feet are then surface measure times the thickness in inches. The council's example is a board 2 inches thick, 6 1/4 inches wide and 8 ft long. It has 6.25 × 8 ÷ 12 = 4.17, so a surface measure of 4, and 4 × 2 = 8 board feet. Two more trade rules change a tally. Lengths are rounded down to the whole foot, so a 9 ft 10 in board sells as 9 ft. Kiln-dried lumber is sold on its dried size, a net tally the council puts about 7 percent under the green tally of the same boards.

A tally for a small table

A dining table about 3 by 5 ft, its top five 8 inch boards trimmed to width, with four legs and four aprons, in 4/4 and 8/4 hardwood:

Part	Size	Count	Board feet each	Total
Top boards	4/4 × 8 in × 60 in (5 ft)	5	3.33	16.7
Legs	8/4 × 3 in × 30 in (2.5 ft)	4	1.25	5.0
Aprons	4/4 × 4 in × 48 in (4 ft)	4	1.33	5.3
Total				27.0

At a dealer's price per board foot the bill is 27 times that price, before the extra for cutting around defects and squaring rough edges. Hardwood dealers sell boards as they come, in random widths and lengths. So the tally is a minimum, and the usual practice is to buy a margin over it. How much depends on the species and the grade, and no published standard sets a percentage.

Logs, plywood and cubic feet

The same words cover two other things. Standing timber and logs are scaled in board feet by log rules. These are estimates of the lumber a log will yield after the saw kerf and slabs are lost. Wisconsin's fact sheet notes a log gives fewer board feet than its solid volume would suggest. Plywood and other sheet goods sell by the sheet and thickness, though a 3/4 inch 4x8 sheet converts to 24 board feet of nominal volume. And a cubic foot of nominal lumber is 12 board feet, since 1,728 cubic inches divided by 144 is 12. The linear feet calculator counts board feet piece by piece for a lumber list. What a linear foot is explains why trim and fence are priced by length rather than by volume.