The tally and the stick are different objects
A board foot is 144 cubic inches: 12 × 12 × 1. For softwood that is at least a nominal inch thick, 7 CFR § 1217.4 says you multiply nominal thickness by nominal width by the actual length in feet, then divide by 12. An 8-foot 2×4 is 5.33 board feet. Ten of them, the list this page opens with, are 53.33 board feet, bought as 10 pieces of 2×4 × 8 feet.
PS 20 is why the stick in your hand is smaller. Table 3’s dressed-dry minimums: nominal 2 inches becomes 1-1/2, nominal 4 becomes 3-1/2, nominal 6 becomes 5-1/2, nominal 8 becomes 7-1/4, nominal 10 becomes 9-1/4, nominal 12 becomes 11-1/4. A 1-inch board dresses to 3/4. A 4×4 is 3-1/2 by 3-1/2. A 6×6, under the timber row that takes 1/2 inch off nominal 5 and 6, is 5-1/2 by 5-1/2. The sheet prints that dressed size, and the board-foot equivalent of the wood actually in the stick, so you can see what planing removed. You still order the nominal tally.
Length is the one actual dimension
Thickness and width are names. Length is what you measure. The Wood Handbook notes that retail construction lumber commonly comes in 2-foot multiples. A line entered at 7 feet becomes an 8-foot purchase, and the board feet jump to the longer stick. If you truly have a mill that will cut 7 feet, set the length to 7 and ignore the stock note; the formula itself did not change, only the assumption about what the rack holds.
Nominal thickness under 1 inch is tallied as 1 inch, again from the CFR, not from PS 20’s dressed column. A 5/4 × 6 is nominal 1.25 inches, which is over the 1 inch line, so it tallies at 1.25. Its dressed thickness in Table 3 is 1 inch.
Mistakes
- Multiplying 1.5 × 3.5 × length ÷ 12 and calling it the order. That is the wood in the stick. The yard did not sell it that way.
- Confusing board feet with linear feet. Ten studs are 80 linear feet and 53.33 board feet.
- Forgetting the ÷ 12, which overstates the order twelve times.
- Adding a waste percent and also rounding every board up, then wondering why the pile is long. The sheet shows both, separately.
- Using green sizes when you are buying kiln-dried S4S. Green dressed sizes in PS 20 are larger. This page uses the dry column because that is the rack at a home-center.
FAQ
Is a 2×4 really 2 inches by 4?
Not after it is dressed dry. PS 20-25 Table 3 puts a dry 2×4 at a minimum of 1-1/2 by 3-1/2 inches. The board-foot tally still uses 2 and 4. Federal rules for the softwood checkoff say thickness and width are nominal and length is actual.
How many board feet is a 2×4×8?
2 × 4 × 8 ÷ 12 = 5.33 board feet. Ten of them are 53.33. Using the dressed 1.5 × 3.5 instead gives 3.5 board feet a stick and under-orders the pile by about a third.
What if the piece I need is 7 feet?
The Wood Handbook says construction lumber is commonly stocked in 2-foot multiples. The sheet rounds the buy length up to the next even foot: 7 feet becomes an 8-foot stick, and the board feet follow the stick you pay for. The cut length stays on the detail line.
Why does a board thinner than 1 inch count as 1 inch?
7 CFR § 1217.4 says softwood lumber with a nominal thickness under 1 inch is calculated as 1 inch. A nominal 3/4 board is still tallied at 1 inch thick. The dressed thickness on the sheet is a different number, for the actual wood, and it is labeled that way.
Sources
- 7 CFR § 1217.4, Board foot — Nominal thickness and width, actual length. Nominal thickness under 1 inch is calculated as 1 inch.
- NIST Voluntary Product Standard PS 20-25 — Table 3, minimum dressed dry sizes. A nominal 2-inch thickness dresses to 1-1/2; a nominal 8-inch width dresses to 7-1/4; a nominal 1-inch board dresses to 3/4.
- USDA Forest Products Laboratory, Wood Handbook, Chapter 6 — Nominal versus dressed sizes, and the note that 2-foot length multiples are common for construction lumber. The PDF filename on the FPL site is spelled documnts.
- David Briggs, lumber measurement chapter — Board feet use nominal size: BF = thickness × width × length ÷ 12. An 8-foot 2×4 is 5.33 board feet, while the manufactured section is smaller.