Board foot defined
August 12, 2010 | 7:00 pm CDT

Q: What is a board foot? Also, I saw something about Doyle board feet -- what are they?

A: Let's take the easy question first. A Doyle board foot (Scribner and International BF are also used) is an estimate of the board feet of lumber in a log. A lumber board foot is technically a piece of lumber that is 1 foot x 1 foot x 1 inch thick. Any volumetric equivalent is also a BF, so a piece 2 inches thick that is 1 x 1/2 foot is also one BF. This is the technical definition. In practice, the softwood board foot and the hardwood board foot are considerably different. This is where it gets a little complicated. In short, always make sure that your supplier and your customer have the same understanding of what a board foot of lumber actually is.

Hardwood BF

In hardwoods, a BF = [SM] x [thickness]. The SM is the surface measure and is equal to [L (feet)] x [W (inches)] / [12] where L = length (4, 5, 6, 16 feet, with no fractions used; do not round-up to the closest foot), and W = width (inches and fractions). The answer for the SM is then rounded to the closest foot, alternating up or down when exactly on the 1/2 foot.

After the SM for each piece is determined, the total SM for each thickness is summed. Then the total is multiplied by the standard thickness (in quarter inches, although under 1 inch is counted as 4/4) to obtain the board feet. Pieces must be as thick or thicker than the standard thickness; for example, 5/4 thickness runs from 1.25 to 1.49 inches and 6/4 goes from 1.50 to 1.74. Note: No allowance is legally permitted for shrinkage (either shrinkage that has happened or that will happen); lumber must be measured and recorded as it is found, not as what it will be or what it was.

Kiln-dried lumber can be 1/16 inch scant if under 8/4 inches and 1/8 inch scant for 8/4 and thicker. For example, 1-7/16 inch thickness is 6/4. Surfaced lumber can be even thinner and still count as a thicker piece. For example, 1-1/16 to 1-5/16 inch actual thickness would be 5/4. Example: Consider 100 pieces of rough, undried hardwood lumber, 6-1/4 inches wide, 12 feet, 8 inches long and 1-1/8 inches thick (called 4/4 thickness). SM of each piece is [6.25] x [12] /[12] = 6 ft. SM (not 6.25). This gives 600 SM for all 100 pieces. Then, BF = [600] x [4/4] = 600 BF. Now, what if the lumber was 1.625-inches thick (called 6/4)? BF = [600] x [6/4] = 900 BF.

How many hardwood BF are in 1 cubic foot of wood? Theoretically, there are 12 BF per CF. In practice, it varies, due to the extra length and the extra thickness that is common in some lumber. For lumber that is 6 inches x 12 feet 8 inches x 1.2 inches, there are 6 BF. But there are 0.633 cubic feet (CF) of wood, so there are 9.5 BF per CF. In other words, with hardwoods, you probably get more wood than you figured!

Softwood BF

In softwoods, we use the same formulae, but we use the nominal dimension for L, W and thickness, not the actual size. Further, we carry the fractions (x.xx), rounding only after summing the SM. Thicknesses, usually after drying, are 4/4 nominal = 0.75 inch; 8/4 (or 2 inch) = 1.5 inches. Actual widths are also smaller than nominal. For example, 4 inch nominal is 3.5 inches actual; 6 is 5.50; 12 is 11.25.

Example: Assume that there are 100 pieces that are 2 x 4 x 10 feet. The actual size is 1.5 x 3.5 x 12. The SM is [4] x [10]/[12] = 3.33 ft SM. The total SM is 333. The BF is [333] x [2] = 666 BF. Question: How many BF in a CF? For a 2 x 4 x 8 (2 inch x 4 inch x 8 foot), there are 5.33 BF. There are 0.29 cu. ft. of wood. Therefore, there are 18.3 BF per CF, considerably more than the theoretical value of 12. So, a softwood BF (18.3 BF per CF) is considerably smaller than a hardwood BF (9.5 BF per CF).

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Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.