What is a board foot? How lumber is measured and priced

The formula

Board feet are thickness × width × length, divided by the volume of one board foot. Because thickness and width are usually in inches and length in feet, there are two versions of the same formula:

board feet = thickness (in) × width (in) × length (in) ÷ 144
board feet = thickness (in) × width (in) × length (ft) ÷ 12

A few worked examples:

A handy shortcut: a 1×12 and a 2×6 both contain exactly one board foot per linear foot. A 1×6 has half a board foot per foot; a 2×12 has two. The board foot chart lists every common size from 1×4 to 6×6 at 8 to 20 ft.

Each of these has a worked page with the price math and nearby sizes: how many board feet in a 2x4x8, how many board feet in a 2x6x8 and how many board feet in a 4x4x8 for construction lumber; board feet in a 4/4 hardwood board, 8 in × 8 ft and board feet in an 8/4 slab, 24 in × 8 ft for hardwood.

Nominal size for softwood, quarters for hardwood

The most common board-foot mistake is plugging in the wrong dimensions. Which size you use depends on how the lumber is sold.

Softwood dimensional lumber

Construction lumber — 2×4s, 2×6s, 4×4s, 1× boards — is named by its nominal size, and board feet are calculated on that nominal size. Under the American Softwood Lumber Standard (NIST PS 20) a 2×4 is dressed to 1½ × 3½ in, but a 2×4×8 is still counted as 5.33 board feet when a yard prices a framing package per thousand board feet. The actual volume is 3.5 board feet; that difference is simply built into the price.

Hardwood

Hardwood lumber is sold by the quarter: its rough-sawn thickness in quarter inches. 4/4 (“four quarter”) is 1 in, 5/4 is 1¼ in, 6/4 is 1½ in, 8/4 is 2 in, and so on. Width and length are measured on each board, because hardwood is sawn to whatever width the log allows. Board feet are figured on the rough thickness — and usually stay on that thickness even if you pay the dealer to surface the boards.

How lumberyards tally hardwood

A hardwood yard does not multiply your exact measurements. Many US dealers follow the National Hardwood Lumber Association (NHLA) rules — illustrated in the American Hardwood Export Council’s grading guide — which work in surface measure: width in inches × length in feet ÷ 12, rounded to a whole number for each board (exactly one half goes up and down alternately). Fractions of a foot of length are dropped, so an 8 ft 6 in board counts as 8 ft. Surface measure is then multiplied by the thickness in inches (so 8/4 counts double) to get board feet, and stock thinner than 1 in is counted as 1 in — a 3/4 in board 6 in wide and 8 ft long is tallied as 4 board feet, not 3.

The upshot: the tally on your receipt can differ from your own careful math by a few percent either way. That is normal. The hardwood tally calculator runs a list of boards the way the yard does and shows the difference; if you are buying a lot, ask how the yard measures before you load the cart.

Prices: per board foot, per MBF, per piece, per linear foot

• Per board foot — the normal unit for hardwood at specialty dealers. Prices depend on species, grade, thickness and whether the boards are rough or surfaced. Thicker stock (8/4 and up) usually costs more per board foot than 4/4 of the same species.
• Per MBF — per thousand board feet, the unit used in wholesale lumber markets and on sawmill and contractor quotes.
• Per piece — how home centers sell framing lumber and boards. Divide the piece price by its nominal board feet to compare with a board-foot quote.
• Per linear foot — common for trim, moulding and some boards. Multiply by 12 and divide by thickness × width (in inches) to convert to a board-foot price.

For a bundle, 100 board feet of 4/4 × 6 in in linear feet shows the conversion from board feet to running length and boards.

Estimating how many board feet a project needs

1. Make a cut list of every part with its finished thickness, width and length.
2. Group parts by thickness. A ¾ in finished part comes from 4/4 stock; a 1¾ in leg from 8/4.
3. Add the board feet for each group using the rough thickness (1 in for 4/4, 2 in for 8/4) and the finished width and length.
4. Add waste. For rough hardwood, 20–30% is a common planning range — more for figured wood, lower grades or lots of short parts.
5. Round up to whole boards when you shop, and pick boards with your longest and widest parts in mind.

Metric equivalents

One board foot is about 0.00236 cubic metres, so one cubic metre is about 423.8 board feet. Outside North America, timber is usually priced per cubic metre or per linear metre at its actual size. North American hardwoods sold abroad are often still quoted in board feet.