Board feet: how to calculate them, and what the lumberyard charges for

Hardwood is sold by volume, and the unit is the board foot. If you can turn a cut list into board feet, you can price a project before you drive to the yard and buy enough wood to finish it without buying twice as much as you need.

The formula

One board foot is 144 cubic inches: a piece 12 inches wide, 12 inches long and 1 inch thick, or any shape with the same volume.

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

With the length in feet, divide by 12 instead:

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

A board 1 inch thick, 6 inches wide and 8 feet long is 1 × 6 × 8 ÷ 12 = 4 board feet. At $8.00 per board foot, it costs $32.00.

What the dealer actually measures

Hardwood dealers measure rough lumber, not the finished part you will make from it, and the thickness they charge for is the nominal rough thickness:

• Thickness is sold in quarters of an inch. 4/4 (“four quarter”) is 1 inch rough, 5/4 is 1-1/4 inches, 8/4 is 2 inches. The board-foot calculation uses those figures even after the board has been surfaced thinner.
• Thin boards count as 1 inch. Under the common hardwood grading rules, lumber thinner than 1 inch is tallied as if it were 1 inch thick.
• Surfacing costs thickness, not money. A 4/4 board surfaced on two sides (S2S) is typically about 13/16 inch thick. You pay for 4/4.

Typical surfaced thicknesses:

Nominal	Rough	Surfaced two sides (typical)
4/4	1”	13/16”
5/4	1-1/4”	1-1/16”
6/4	1-1/2”	1-5/16”
8/4	2”	1-3/4”

Yards differ, so ask what they surface to. Softwood dimensional lumber (2x4s and the like) is usually priced per piece or per linear foot instead, and sheet goods by the sheet; see nominal and actual sizes.

Choosing the thickness to buy

Pick the thinnest nominal thickness that will still mill flat to your finished size:

• 3/4” finished parts: buy 4/4. It leaves about 1/4 inch to flatten a board that is slightly cupped or twisted.
• 1” finished parts: buy 5/4.
• 1-1/2” legs: buy 8/4, or glue up two pieces of 4/4 milled to 3/4 inch. The glue-up is cheaper but shows a glue line on two faces.

A thicker board milled down to a thin part is money spent on shavings. A board too close to finished thickness may not clean up at all.

From cut list to lumber order

1. Start from finished sizes. List every solid-wood part with its finished thickness, width and length.
2. Group by thickness and species. Each group becomes one line on the order.
3. Add milling allowances to each part: commonly 1/4 to 1/2 inch in width and 1 to 2 inches in length, more for long parts.
4. Convert to board feet at the nominal thickness you will buy.
5. Add a waste factor for knots, checks, sapwood, grain matching and the offcuts you cannot use. Many woodworkers add 20 to 40 percent; go toward the high end for lower grades, rough stock that needs heavy milling, or careful grain matching on show surfaces.

A worked example

Four legs, each 1-1/2 × 1-1/2 × 28 inches finished, from 8/4 stock:

• Blank per leg with allowance: 2 (nominal) × 2 × 30 ÷ 144 = 0.83 board feet.
• Four legs: 3.33 board feet.
• With 30 percent waste: about 4.3 board feet.

Leg blanks this narrow are usually ripped from a wider board, so buy a board that yields four blanks side by side or end to end, plus saw kerfs.

Net and gross board feet

A board-foot figure calculated from finished dimensions is the net amount of wood that ends up in the piece. It is useful for comparing designs and species, but it is not a shopping list. GrainDraft’s stock list, for example, reports board feet from each part’s finished size and prices it at the rate you enter; for the starter side table that is 7.34 board feet. Treat a number like that as net and apply the allowances above before you order.

Pricing checklist

• Confirm whether the price is per board foot, per linear foot or per piece.
• Ask whether the quoted price is for rough or surfaced lumber, and what the surfaced thickness will be.
• Check the grade. FAS (firsts and seconds) yields long, clear cuttings; common grades cost less and yield shorter clear pieces.
• Buy a little extra for test cuts. A joint tried in scrap from the same board is worth its cost.

Related: wood movement, sizing a mortise and tenon.