How to calculate board feet
A board foot is a slab of wood 1 inch thick and 1 foot square: 144 cubic inches. Lumber yards use it to price wood of any size on one scale. Here is the math with the list the calculator opens with: twelve 2x4s 8 ft long, four 2x6s 12 ft long and two 4x4 posts 8 ft long, framing lumber at $0.85 a board foot.
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1
Multiply thickness x width x length
Use the nominal size in inches and the length in feet. For a 2x4x8: 2 x 4 x 8 = 64.
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2
Divide by 12
64 ÷ 12 = 5.33 board feet per stud. (With the length in inches, divide by 144 instead.)
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3
Multiply by the count
12 studs x 5.33 = 64 board feet. The four 2x6x12s are 12 each, or 48, and the two 4x4x8 posts are 10.67 each, or 21.33.
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4
Add the rows
64 + 48 + 21.33 = 133.3 board feet, in 18 pieces and 160 linear feet.
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5
Price it
133.3 x $0.85 = $113. If your store lists a price per piece, type it in the "$ each" box and the calculator uses that instead.
Switch the extra to 10% and the list grows to 14 studs, 5 2x6s and 3 posts, because extra pieces are rounded up row by row. In metric mode the calculator shows each board's real size in millimeters, lengths in meters and the total in cubic meters (1 m³ = 423.8 board feet).
Nominal vs. actual lumber sizes
A 2x4 left the sawmill close to 2 by 4 inches. Drying shrinks it and planing smooths every face, so the finished board is 1½ by 3½ inches. The American Softwood Lumber Standard fixes these dressed sizes, which is why every 2x4 in the country matches. Board feet are still counted on the nominal size.
| Nominal | Actual, in | Actual, mm | Bd ft per ft |
|---|---|---|---|
| 1x4 | ¾ x 3½ | 19 x 89 | 0.33 |
| 1x6 | ¾ x 5½ | 19 x 140 | 0.50 |
| 5/4x6 deck | 1 x 5½ | 25 x 140 | 0.63 |
| 2x4 | 1½ x 3½ | 38 x 89 | 0.67 |
| 2x6 | 1½ x 5½ | 38 x 140 | 1.00 |
| 2x8 | 1½ x 7¼ | 38 x 184 | 1.33 |
| 2x10 | 1½ x 9¼ | 38 x 235 | 1.67 |
| 2x12 | 1½ x 11¼ | 38 x 286 | 2.00 |
| 4x4 | 3½ x 3½ | 89 x 89 | 1.33 |
| 6x6 | 5½ x 5½ | 140 x 140 | 3.00 |
The rule of thumb: boards 1 inch thick lose ¼ inch, 2 to 6 inch widths lose ½ inch, and 8 inch and wider boards lose ¾ inch. Lengths are not trimmed: an 8 ft board is at least 8 ft long, often a touch more. Precut studs are the exception, at 92⅝ or 104⅝ inches so that a wall with three plates comes out at 8 or 9 ft.
The gap matters when you plan. Three 2x4s laid flat are 4½ inches, not 6. A deck framed with 2x8 joists is 7¼ inches deep, not 8. And a 2x4 holds only about two-thirds of the wood its name suggests, which is why a board foot of framing lumber is not as big as it sounds.
Board feet in common pieces
| Piece | Board feet | At $0.85 |
|---|---|---|
| 2x4, 8 ft stud | 5.33 | $4.53 |
| 2x6, 8 ft | 8.00 | $6.80 |
| 2x8, 12 ft | 16.00 | $13.60 |
| 2x10, 16 ft | 26.67 | $22.67 |
| 2x12, 16 ft | 32.00 | $27.20 |
| 4x4, 8 ft post | 10.67 | $9.07 |
| 6x6, 8 ft post | 24.00 | $20.40 |
Big-box stores price softwood per piece, and the per-board-foot price is not flat: studs are often the cheapest wood in the store per board foot, while long 2x12s and wide boards cost more. Use the board foot rate for a quick budget, then type real shelf prices into "$ each" for the rows that matter most.
What lumber costs in 2026
A national price survey from October 2026 put an 8 ft 2x8 at about $9 in untreated SPF, $16 pressure-treated and $34 in cedar. That works out to about $0.85, $1.50 and $3.20 per board foot, which are the calculator's defaults. Ranges are wide: the same survey lists $6 to $14 for that 2x8 in SPF. Treated lumber rated for ground contact costs more than the above-ground grade, and clear (knot-free) cedar can cost twice as much as knotty. Prices follow the lumber futures market, so a quote from last spring may be stale.
Hardwood is measured differently
Hardwood dealers sell rough lumber by thickness in quarter inches: 4/4 is 1 inch, 5/4 is 1¼, 8/4 is 2 inches. Boards come in random widths and lengths, so the yard measures each one and charges by the board foot, using the rough thickness. Planed hardwood is thinner than its name, just like a 2x4: 4/4 stock usually surfaces to about ¾ inch. For a hardwood pile, enter each board as a 1x with its width rounded to a size in the list, or add up the board feet by hand with the same formula.
Tips for buying lumber
- Sight down every board. Hold one end at eye level and look along the edge. Leave boards that twist, cup or bow badly; a gentle crown is normal, and it goes up on joists and rafters.
- Buy the right treatment. Pressure-treated wood is labeled for above-ground use or ground contact. Posts, and anything within 6 inches of the soil or hard to replace, should be rated for ground contact.
- Use hardware made for treated wood. The copper in treated lumber corrodes plain steel. Use hot-dip galvanized or stainless screws, nails and hangers.
- Let wet lumber dry. Lumber stamped S-DRY or KD has been dried to 19% moisture or less. Wet treated wood shrinks as it dries, so space deck boards with that in mind.
- Buy long, cut short. One 16 ft board often costs less than two 8 ft boards, and gives fewer joints. Check whether your offcuts can become blocking.
- Check what the span tables say. Joist, rafter and beam sizes for decks and floors come from span tables in the building code, not from board feet. Our deck calculator and stair calculator handle those layouts, and a permit office can tell you what your town requires.
Common questions
What is a board foot?
A board foot is a volume of wood 1 inch thick, 12 inches wide and 12 inches long, or 144 cubic inches. For softwood lumber it is figured from the nominal size, so a 2x4 counts as 2 by 4 inches even though it measures 1.5 by 3.5.
How many board feet are in a 2x4x8?
5.33 board feet. Multiply 2 x 4 x 8 to get 64, then divide by 12. A 2x6x8 is 8 board feet and a 4x4x8 is 10.67.
Why is a 2x4 not 2 inches by 4 inches?
The name is the nominal size of the rough-sawn board. Drying and planing the faces smooth take it down to 1.5 by 3.5 inches. The American Softwood Lumber Standard sets those dressed sizes, so every 2x4 sold in the US is the same.
How do you calculate board feet?
Multiply thickness in inches by width in inches by length in feet, then divide by 12. If the length is in inches, divide by 144 instead. Multiply by the number of pieces for the total.
How much does a board foot of lumber cost?
In late 2026, common framing lumber works out to roughly 75 cents to $1.25 per board foot at retail, pressure-treated to about $1.25 to $2, and cedar to about $3 or more. Prices move with the lumber market, so check your local yard.
How much extra lumber should I buy?
Add about 10% for framing to cover cuts, mistakes and crooked boards you reject. For trim and decking with many angled cuts, 15% is safer. Pick through the pile and skip boards that are twisted or badly bowed.
Sources and assumptions
Board feet are plain arithmetic on nominal sizes. Prices are US retail figures checked in October 2026 and can all be edited.
- Dressed sizes (1½ x 3½ for a 2x4, 1½ x 7¼ for a 2x8 and so on, dry lumber) follow the American Softwood Lumber Standard, NIST PS 20, published by the American Lumber Standard Committee, and the section property tables in the American Wood Council NDS Supplement. Millimeter sizes are the inch sizes converted and rounded.
- Board foot: 144 cubic inches, nominal thickness x nominal width x length in feet ÷ 12, as defined in the USDA Forest Products Laboratory Wood Handbook. 1 m³ = 423.776 board feet.
- Prices: 2x8x8 at about $9 (SPF, range $6–$14), $16 (pressure-treated) and $34 (cedar), and $0.75–$2.00 per linear foot for SPF, from LatestCost, 2x8 lumber prices (October 2026). Our per-board-foot defaults ($0.85, $1.50, $3.20) are those piece prices divided by 10.67 board feet.
- Moisture: "dry" softwood lumber is 19% moisture content or less under PS 20. Extra pieces are rounded up per row, and the cost is board feet x rate unless you type a price per piece.
- Structure: board feet say nothing about strength. Decks, floors, beams and anything that holds people up must follow the span tables of your local building code (the IRC in most US towns).