Hardwood is sold by the board foot, and it is the one lumber measurement that confuses almost everyone at first. Unlike a 2x4 stud — which you buy by the piece — hardwood boards are priced by volume: one board foot equals 144 cubic inches of wood, traditionally pictured as a board 12 inches wide, 12 inches long, and 1 inch thick. Once you know the formula, you can price any board in seconds and check any lumber quote with confidence.
The formula is simple: board feet = (thickness in inches × width in inches × length in feet) ÷ 12. Every dimension goes in as stated on the nominal size, and the length stays in feet. A 2x4 that is 8 feet long comes out to (2 × 4 × 8) ÷ 12 = 64 ÷ 12 ≈ 5.33 board feet. A 1x6 that is 10 feet long is (1 × 6 × 10) ÷ 12 = 60 ÷ 12 = 5 board feet exactly. If you prefer to work entirely in inches, use the equivalent form: (thickness × width × length in inches) ÷ 144 — the 1x6x10 board is (1 × 6 × 120) ÷ 144 = 720 ÷ 144 = 5 board feet, the same answer.
Here is the trap: nominal sizes are not actual sizes. A '2x4' is really 1.5 by 3.5 inches; a '1x6' is really 0.75 by 5.5 inches. The wood loses size in drying and planing, and the nominal name sticks around anyway. For board-foot pricing, the industry convention is to use the NOMINAL dimensions — the 2x4x8 is sold as 5.33 board feet, not the 4.67 board feet its actual dimensions would give. This is standard practice at lumberyards, not a scam, but it is worth knowing so your own math matches the invoice. The Board Foot Calculator uses nominal dimensions for exactly this reason.
To estimate a small project, work board by board and add. Say you are building bookshelves and need 12 boards of 1x10, each 4 feet long. One board is (1 × 10 × 4) ÷ 12 = 40 ÷ 12 ≈ 3.33 board feet. Twelve of them give 12 × 3.33 = 40 board feet. Add a waste allowance — 10% is reasonable for simple straight cuts, 15–20% if the design has many angles or you are new to the work — so 40 × 1.10 = 44 board feet to buy. Lumberyards sell random-width boards by the board foot too, in which case you estimate each board's width and add up the footage the same way.
Cost estimation follows directly. Multiply the total board feet by the price per board foot, which the yard quotes for the species and grade you want. At $6.50 per board foot, our 44-board-foot shelf project costs 44 × $6.50 = $286 in lumber. Prices vary enormously by species — poplar might be $3–4 per board foot while walnut runs $10–15 or more — so always confirm the per-board-foot price for your exact species and grade before multiplying. And remember that surfaced (planed) lumber sometimes carries a different rate than rough stock.
A few practical notes from the lumber aisle. First, boards are sold in even and odd foot lengths, so your actual purchase will round up — if the math says 44 board feet but the available boards total 47, you pay for 47. Second, check the grade: cheaper grades have more knots and defects, which effectively raises your waste factor. Third, for softwood framing lumber (2x4s, 2x6s), ignore board feet entirely — it is sold by the piece, and the board-foot math only matters for hardwoods and specialty boards.
The whole system exists so that odd-sized boards can be priced fairly against each other. A 1x8x12 and a 2x4x12 are both 8 board feet, and at the same per-board-foot price they cost the same — which is exactly right, because they contain the same volume of wood. Learn the one formula, remember that nominal dimensions rule, and the lumberyard stops being a place where you hope the math was done right.