The board-foot formula, without the lumberyard confusion
A board foot is a measure of wood volume. One board foot is a board that is 1 inch thick, 12 inches wide, and 12 inches long. It is the unit many hardwood dealers use when they price lumber.
Use this formula when thickness and width are in inches and length is in feet:
Board feet = thickness × width × length ÷ 12
If all three measurements are in inches, divide by 144 instead:
Board feet = thickness × width × length ÷ 144
That is the math. The part worth slowing down for is making sure your measurements match the way the lumberyard tallies the board.
Work one board first
Say you find a 4/4 board that is 6 1/2 inches wide and 8 feet long. Using 1 inch as the rough 4/4 thickness:
1 × 6.5 × 8 ÷ 12 = 4.33
That board contains 4.33 board feet under that measurement basis. If the board costs $8 per board foot, the rough material estimate is 4.33 × $8, or $34.64 before tax and any yard-specific charges.
Do not assume every board marked 4/4 measures a full inch after surfacing. A yard may price rough, surfaced, or pre-tallied stock differently. Ask which dimensions and tally method they use before you compare your estimate with the invoice.
Add up several boards
For identical boards, calculate one and multiply by quantity. Five 1-inch-thick boards that are 6 inches wide and 8 feet long work out like this:
1 × 6 × 8 ÷ 12 = 4 board feet per board
4 × 5 = 20 board feet total
For mixed boards, calculate each line separately and add the results. For example:
| Board | Calculation | Board feet |
|---|---|---|
| 1 × 7 1/4 in. × 6 ft. | 1 × 7.25 × 6 ÷ 12 | 3.63 |
| 5/4 × 8 in. × 5 ft. | 1.25 × 8 × 5 ÷ 12 | 4.17 |
| 8/4 × 5 in. × 4 ft. | 2 × 5 × 4 ÷ 12 | 3.33 |
The order is about 11.13 board feet. Round your estimate only after you have totaled the boards, then check how the seller rounds its tally.
If you already have a project plan, make a board list before you buy. A woodworking cut list tells you what parts you need; board-foot math tells you the volume of lumber you are considering. They solve different problems, and using both saves a lot of aisle-side arithmetic.
Rough, nominal, and actual dimensions are not interchangeable
This is where first lumberyard trips get expensive.
Hardwood thickness often appears in quarters. Four-quarter, written 4/4, starts at roughly 1 inch thick before surfacing. Five-quarter is roughly 1 1/4 inches, and eight-quarter is roughly 2 inches. Planing and jointing remove material, so a surfaced board can end up thinner than its rough designation.
Softwood dimensional lumber creates a different kind of confusion. A home-center 2×4 does not physically measure 2 by 4 inches after it is dried and surfaced. Its nominal name and actual size serve different purposes. Use the size basis your seller uses for pricing, but use the board's real, current dimensions when you need to know whether a finished part will fit.
The safest question at the counter is simple: “Are you charging board feet from the rough size, the surfaced size, or a yard tally?” Then write the answer beside your estimate. For more help translating a plan's dimensions, read how to read a woodworking cut list and understand nominal versus actual lumber sizes.
A board foot is not a linear foot or a square foot
A linear foot measures length only. A square foot measures area. A board foot includes thickness, width, and length, so it measures volume.
That distinction matters when you compare boards. An 8-foot-long, 4-inch-wide board and an 8-foot-long, 8-inch-wide board have the same linear footage, but the wider board contains twice as much material at the same thickness.
Plan for usable lumber, not only the formula
Board-foot math tells you the volume of the boards. It does not promise that every inch will become a finished part. Knots, checks, wane, grain direction, color matching, and saw kerfs all change what you can use.
For a small, simple project with straight, surfaced boards, a modest extra allowance may be enough. Give yourself more room when you are working with rough stock, long parts, highly figured boards, or a cut list that leaves little flexibility. A compact book-valet-style project can look small on paper but still need extra length for trimming ends and choosing the best grain.
Lay out your longest parts first, then see what remains for shorter pieces. If a board is beautiful but cannot yield the part you need, it is not a bargain.
Take this checklist to the lumberyard
Before you buy, bring your cut list and note:
- Species and thickness you need.
- Finished part sizes and the extra material needed for milling.
- Board-foot price or price-per-board, plus how the yard calculates its tally.
- Whether the stock is rough, S2S, or S4S.
- The longest part in your project.
- Any defects, grain requirements, or color matching that will reduce usable yield.
Choose boards with the project in mind, not just the lowest number on the rack. If you are still deciding on species, this guide to choosing wood for woodworking projects can help you match the material to the job before you start calculating.
Once you can estimate board feet, lumber quotes get much easier to compare. More importantly, you can walk into the yard with a plan, ask the right question about the tally, and leave with enough wood to build the project.