Board Foot Calculator

The formula

A board foot is a volume: 144 cubic inches, which is the space taken up by a piece one inch thick, twelve inches wide and one foot long. It is not an area, and it is not a length. Lumber has been sold this way for two centuries because it prices wood by how much wood there is, regardless of what shape it arrives in.

Two forms of the same calculation, depending on which unit your length is in:

Board feet = thickness (in) × width (in) × length (in) ÷ 144

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

Multiply by the number of pieces for the total. That is the whole thing.

A worked example

Ten pieces of 4/4 hardwood, 6 inches wide, 8 feet long, at $7.50 a board foot, with 15% allowed for cutting waste.

• Per piece: 1 × 6 × 96 ÷ 144 = 4.0 bd ft
• Ten pieces: 40.0 bd ft
• With 15% waste: 46.0 bd ft
• Cost: 46 × $7.50 = $345

Now the same pieces in 8/4 stock — two inches thick instead of one. Per piece: 2 × 6 × 96 ÷ 144 = 8.0 bd ft, so the pile doubles to 80 bd ft and $690 with waste. Thickness scales the price linearly, which is why designing a project in 4/4 rather than 8/4 where you can is the cheapest decision you will make.

The quarter system, which is how yards actually talk

Hardwood thickness is quoted in quarters of an inch, written as a fraction over 4:

• 4/4 — "four quarter" — 1 inch rough
• 5/4 — 1¼ inch
• 6/4 — 1½ inch
• 8/4 — 2 inch
• 12/4 — 3 inch
• 16/4 — 4 inch

The quarter figure is the rough thickness, straight off the mill before anything is surfaced. Once it is planed on both faces — S2S — a 4/4 board typically arrives at 13/16 in or ¾ in. And here is the part that costs money: most yards tally rough hardwood at the nominal quarter thickness even after surfacing. You pay for 4/4 and you receive 13/16. That is the trade convention, not a mistake, and it is why buying surfaced stock and buying rough stock at the same board-foot price are not the same deal.

Width is measured to the nearest inch on rough hardwood, and the yard usually rounds partial inches to their advantage on random-width stock. Length is measured to the foot.

The nominal trap in softwood

Softwood construction lumber works differently, and mixing the two systems produces board-foot numbers that are quietly 30 or 40 percent wrong.

A 2×4 is not two inches by four inches. It is 1½ by 3½. A 2×6 is 1½ by 5½, a 1×6 is ¾ by 5½, a 2×10 is 1½ by 9¼. The nominal figure describes the rough green size before drying and surfacing; the actual figure is what you get.

Run an 8 ft 2×4 both ways:

• Nominal: 2 × 4 × 8 ÷ 12 = 5.33 bd ft
• Actual: 1.5 × 3.5 × 8 ÷ 12 = 3.50 bd ft

That is a 52 percent overstatement. If you are estimating a framing package by board foot from nominal sizes, you are budgeting for half as much wood again as will arrive on the truck.

The confusing part is that the lumber trade itself often quotes softwood in nominal board feet, because that is how mill output has always been reported. So the honest answer is: ask which basis the price is on. This calculator gives you both — switch the sizing selector and compare.

Where board feet matter and where they do not

Board feet are the right unit for hardwood, rough stock, slabs, and anything sold by volume — furniture and cabinet work, essentially. If you are buying random-width, random-length cherry for a table, board feet is the only sensible way to price it.

They are the wrong unit for plywood and sheet goods, sold by the sheet; for trim and moulding, sold by the lineal foot; and for dimensional framing lumber, which most yards price by the piece at a given length. Nobody buys studs by the board foot.

Waste, which is larger than you think on hardwood

Fifteen percent is the working minimum on rough hardwood, and twenty to thirty is realistic when you are working around knots, checks, sapwood and cupping, or when your cut list wants specific widths from random-width stock. Rough lumber loses thickness to jointing and planing, width to edge-jointing, and length to defect cutting — and then the piece you wanted to be 5 inches wide has to come out of a 6 inch board.

Buy the whole project at once, from the same lot if you can. Colour varies between trees, and matching a board eight months later is unlikely.