Board Foot Calculator

What a board foot actually measures

A board foot is a unit of volume, not of length or of surface. One board foot is 144 cubic inches: a piece one inch thick, twelve inches wide and one foot long. Every hardwood yard in the United States, and most in Canada, prices its stock this way, because it is the only measure that stays fair across the random widths and lengths that come off a log. The formula is thickness in inches times width in inches times length in feet, divided by twelve.

That division by twelve is where most people go wrong. Length is in feet while the other two dimensions are in inches, so the twelve converts feet to inches without you having to write out 144 cubic inches every time. Get the units mixed up — length entered in inches, say — and your total will be twelve times too big.

Worked example: ten boards of 4/4 walnut

You want ten pieces of 4/4 walnut, 6 inches wide and 8 feet long. Thickness is 1 inch, so one board is 1 x 6 x 8 / 12 = 4 board feet. Ten pieces gives 40 board feet. At $4.50 per board foot the lumber comes to 80.00 before tax and before any delivery charge. Notice that the per-piece figure is the useful one when you are standing at the rack: anything 6 inches wide and 8 feet long in 4/4 is 4 board feet, so you can total a cart in your head.

Switch that order to 8/4 and the arithmetic doubles: 2 x 6 x 8 / 12 = 8 board feet per piece, 80 board feet total, $360.00 at the same price. Thickness is the most expensive variable in any hardwood order, which is why yards quote 8/4 stock at a higher per-board-foot rate on top of the doubled volume.

The quarter system

Hardwood thickness is quoted in quarters of an inch, measured on the rough board before it is planed. Say it out loud the way the yard does: four-quarter, five-quarter, eight-quarter.

The surfaced-lumber trap

This is the single most common mistake in checking a lumber invoice. A 4/4 board surfaced on two sides arrives at roughly 13/16 inch thick, and you measure it, enter 0.8125 in the calculator, and conclude the yard overcharged you by about twenty percent. It did not. Hardwood is tallied at the rough size before surfacing, and width is measured before the edges are straight-lined. Enter the nominal figures — the ones on the quote — and the numbers will match.

Softwood construction lumber works differently again. A 2x4 is nominally two inches by four inches but actually measures 1.5 by 3.5 inches, and it is usually sold by the piece or by the linear foot rather than by board foot. If you do need board feet for framing material, use the nominal 2 and 4, not the actual dimensions.

Estimating a real project

Work out the finished parts first, then add for waste. Take a small side table: four legs at 8/4, 2 inches square and 28 inches long, plus a top of 4/4 stock 18 by 30 inches. The legs come to roughly 2 x 2 x 2.33 / 12 = 0.78 board feet each, so about 3.1 board feet for four. The top is 1 x 18 x 2.5 / 12 = 3.75 board feet. That is just under 7 board feet of finished parts.

Now add the waste allowance. Fifteen to twenty percent covers ordinary work: saw kerf, jointing losses, trimming ends square. Push it to thirty percent if you are matching grain across a panel, cutting around knots and sapwood, or making a lot of short parts where board ends are unusable. For this table you would buy around 9 board feet, and you would round up to whatever the yard's boards actually give you, since you cannot buy a fraction of a plank.

One last practical note: pricing is normally per board foot and varies by species, grade and thickness, and many yards apply a minimum order or a surcharge for widths over ten inches. Use the cost figure here as a planning number, then confirm the grade and the surfacing charge before you place the order.