construction
How to Calculate Board Feet (and Why a 2x4 Isn't 3.5 of Them)
Board feet = thickness x width x length / 144. Softwood is tallied on nominal size, so a 2x4x8 counts as 5.33 BF but holds 3.5. Worked table inside.
Pick up a 2x4 that is 8 feet long and measure it. You get 1½ by 3½ inches. Run those numbers through the board-foot formula and you get 3.5 board feet. Now look at how the lumber trade counts it: 5.33 board feet. Both numbers are correct.
A board foot is a pricing and tally unit built on nominal sizes, not a measurement of how much wood is in your hands. The difference is about a third of the volume on small framing lumber, and it explains why calculators online seem to disagree with each other.
Below: the formula, the rule for which dimensions to plug in, a table of common sizes showing both numbers, how hardwood “quarters” work, and a cost example. The math matches our board foot calculator, which takes thickness, width, length, piece count and an optional price per board foot.
The formula
A board foot is 144 cubic inches: a piece 1 inch thick, 12 inches wide and 12 inches long, or any shape with the same volume. The USDA Forest Products Laboratory’s Wood Handbook defines it exactly that way.
board feet = thickness (in) × width (in) × length (in) ÷ 144
board feet = thickness (in) × width (in) × length (ft) ÷ 12Use the first line if you measured the length in inches, the second if you measured it in feet. They are the same formula, because 144 ÷ 12 = 12.
Worked example. A 2x6 that is 10 feet long:
2 × 6 × 10 ÷ 12 = 10 board feetFor a stack of identical pieces, work out one piece and multiply by the count. Ten of those 2x6s is 100 BF. Large orders are often quoted per MBF, which is a thousand board feet.
Nominal or actual: which numbers go in?
This is the part the top search results contradict each other on. The answer depends on the kind of lumber.
Softwood (framing, dimension lumber, common boards): use nominal thickness and width, actual length. The Wood Handbook glossary says it plainly: “In practice, the board foot calculation for lumber 1 in. or more in thickness is based on its nominal thickness and width and the actual length.” It adds a second rule most people never hear about: lumber with a nominal thickness under 1 inch is counted as 1 inch.
The nominal name is not the size of the board. The American Softwood Lumber Standard (PS 20) sets the actual dressed sizes, and the Wood Handbook reproduces them in its Table 6-6. For dry lumber:
| Nominal | Actual (dry, dressed) |
|---|---|
| 1 | ¾” |
| 2 | 1½” |
| 3 | 2½” |
| 4 | 3½” |
| 6 | 5½” |
| 8 | 7¼” |
| 10 | 9¼” |
| 12 | 11¼” |
So the pattern is: a 1x board loses ¼”, sizes from 2 to 7 inches lose ½”, and 8 inches and up lose ¾”. Length is not shrunk. An 8-foot board is 8 feet.
Rough hardwood: use the rough thickness and the measured width. Hardwood is sawn to random widths, so there is no nominal width to fall back on. You measure each board. Thickness is quoted in quarters, covered below.
Nominal vs actual board feet for common sizes
Here is the gap in numbers. “Nominal BF” is what a lumberyard tallies. “Actual BF” is the wood you are holding, using the PS 20 dry dressed sizes.
| Piece | Actual size | Nominal BF | Actual BF | Actual as % of nominal |
|---|---|---|---|---|
| 1x4 x 8’ | ¾” × 3½” | 2.67 | 1.75 | 65.6% |
| 1x6 x 8’ | ¾” × 5½” | 4.00 | 2.75 | 68.8% |
| 2x4 x 8’ | 1½” × 3½” | 5.33 | 3.50 | 65.6% |
| 2x4 x 10’ | 1½” × 3½” | 6.67 | 4.38 | 65.6% |
| 2x6 x 8’ | 1½” × 5½” | 8.00 | 5.50 | 68.8% |
| 2x6 x 12’ | 1½” × 5½” | 12.00 | 8.25 | 68.8% |
| 2x8 x 10’ | 1½” × 7¼” | 13.33 | 9.06 | 68.0% |
| 2x10 x 12’ | 1½” × 9¼” | 20.00 | 13.88 | 69.4% |
| 4x4 x 8’ | 3½” × 3½” | 10.67 | 8.17 | 76.6% |
| 4/4 x 6” x 8’ (hardwood) | 13/16” S2S × 6” | 4.00 | 3.25 | 81.3% |
| 8/4 x 6” x 8’ (hardwood) | 1¾” S2S × 6” | 8.00 | 7.00 | 87.5% |
Two things stand out. Small softwood sizes lose the most, around a third, because ½” off a 2-inch dimension is a big fraction. And the hardwood rows lose less, because only the thickness is planed down in this example and the width was measured, not nominal.
Neither number is wrong. Use nominal BF when you are comparing prices or checking an invoice. Use actual BF if you are estimating weight or the real volume of material, for example to work out what a load weighs.
How hardwood quarters work
Rough hardwood thickness is written in quarters of an inch. The American Hardwood Export Council’s measurement guide lists the standard rough thicknesses:
| Quarters | Rough thickness |
|---|---|
| 4/4 | 1” |
| 5/4 | 1¼” |
| 6/4 | 1½” |
| 8/4 | 2” |
| 10/4 | 2½” |
| 12/4 | 3” |
| 16/4 | 4” |
You say “four quarter” for 4/4 and “eight quarter” for 8/4. Plug the rough thickness into the formula, so 8/4 is 2. A walnut board that is 8/4, 7 inches wide and 6 feet long is 2 × 7 × 6 ÷ 12 = 7 BF.
What you actually get after planing is thinner. The Wood Handbook (Table 6-4) lists 1” rough as 13/16” surfaced on two sides (S2S), and 2” rough as 1¾”. AHEC gives the rule of thumb: subtract 3/16” for stock 1½” and thinner, and ¼” for 1¾” and thicker. Hardwood bought S2S is generally still tallied at its rough thickness, so check with the yard which basis their price uses.
Two more hardwood details from the AHEC guide are worth knowing before you compare quotes:
- Surface measure (SM) is width (in) × length (ft) ÷ 12, rounded to a whole number. It is the face area in square feet, and the grade rules are based on it. For 4/4 stock, SM and BF are the same number.
- Net vs green tally. Kiln-dried lumber can be sold on the board feet measured after drying (net tally) or before drying (gross or green tally). On a green tally, AHEC says to expect roughly 7% fewer board feet in what you receive, because of shrinkage in the kiln.
What does it cost?
Board feet only matter because they turn into dollars. Say you are building a tabletop from 8/4 walnut and need 8 boards, each 7 inches wide and 6 feet long, and the yard charges $12 per board foot (a made-up price for the example; check your local yard).
per board: 2 × 7 × 6 ÷ 12 = 7 BF
total: 7 × 8 = 56 BF
cost: 56 × $12 = $672The same price expressed per linear foot is 2 × 7 ÷ 12 × $12 = $14 per foot, which is handy when one supplier quotes by the BF and another by the foot. If you plan for waste (knots, checks, snipe, grain you want to skip), add your allowance before you multiply. At 25% extra, 56 BF becomes 70 BF and $840.
The board foot calculator does this in one pass. Type 8/4 in the thickness box (or tap a quarter chip), enter the width, length and piece count, and add a price per board foot. It shows the total BF, BF per piece, linear feet, total cost and the implied price per linear foot. Length accepts entries like 8' 6" or 2.4m. Its linear-to-board-feet converter covers common nominal sizes from 1x2 to 6x6, and it includes the nominal vs actual size chart from PS 20.
One thing the calculator does not do is apply the under-1-inch rule. It reports the exact board footage of what you type. If your supplier tallies ½” stock as 1”, enter 1 as the thickness.
Board feet vs linear feet
Linear feet is just length. Board feet folds in the cross-section. To convert, find the board feet in one linear foot of that size:
BF per linear foot = thickness × width ÷ 12 (nominal inches)A 2x4 is 0.667 BF per foot, a 2x6 is exactly 1, a 2x12 is 2 and a 1x6 is 0.5. So 100 linear feet of 2x4 is 66.7 BF.
Related tools
Board feet tell you what a load of lumber costs. They do not tell you how many pieces a project needs. For that, the deck calculator turns a deck size into decking boards, joists, beams, screws and footings, and the fence calculator counts posts, rails, pickets and concrete bags. For metric conversions of any dimension, use the unit converter. If your project also needs gravel, soil or concrete, the same “price by volume” logic applies, covered in cubic yards, tons and density.
Before you head to the yard, run your cut list through the board foot calculator with your price per BF. Then ask one question at the counter: is that price on nominal, rough or surfaced size?