Board Foot Measure

Board foot measure is used to communicate wood volume in the manufacture and wholesale of sawn lumber wood products and products derived from sawn lumber such as glued laminated timbers. In short, one board foot of lumber is equivalent to one 1 x 12 (12 square inches) that is one foot (ft) long. The `1′ and the `12′ are in inches. So, one 2 x 6 one foot long is also one `board foot’ of volume measure (2 times 6 equals 12). Easy enough! A 2 x 6 that is 10 ft long has a board foot volume of 10 (10 B.F. or 10 BF). One hundred of such 2 x 6’s have a volume of 1000 B.F., or 1 `thousand board feet’ (1 MBF). And, in fact, lumber is sawn, sold, distributed, etc. in such volumes that the common unit in communicating such is `thousand board feet’, or, actually, often just `thousand’, with the `board feet’ part implied. So, for example, when discussing the price of thousands of board feet of 2 x 6s it would be common to state a dollar price `per thousand’.

Let us now consider the `board feet’ of lumber joists that will go into a floor that is framed with 2 x 12s spaced 16 inches (in.) on center (o.c.) spanning 16 ft across a floor that is 30 ft wide. One way to go about this would be to calculate the total width of the floor in inches, in this case 30 ft times 12 in. per ft = 360 inches. Then, to get the number of joists we could divide 360 in. by 16 in. per joist to get 22.5 joists. Well, we really wouldn’t use half (.5) of a joist; we would probably use just 22, and the last space at one end would be half of 16 or 8 in., so, 23. But we also need to start with a joist, so, 24. (You may want to sketch this out!) Each individual joist is 2 x 12 divided by 12 equals 2 board foot per foot of length. And since each joist is 16 ft long, each joist is 2 board feet per foot times 16 feet equals 32 board feet. Thus, to frame the floor, 24 joists times 32 board feet per joist gives 768 BF (almost a `thousand’). If these 2 x 12s are part of a large project (including wood volumes for other floors, walls, roof, etc.) their volume might be ordered and priced in terms of (thousands of) board feet. Or, they might be ordered and priced by `linear feet’. The floor in question, for example, would have 24 times 16 equals 384 linear feet of 2 x 12s. Indeed, if we go into the local building supply store and order just one or several 2 x 12s they will be priced by (linear) foot, and the price will vary by wood species, grade, how long, and maybe how many if the `several’ is enough to motivate a discount. The `how long’ comes into play as the longer length lumber pieces (longer than 12 ft or 16 ft) generally cost a bit more per foot.

The conversation could end here, except that mention has also been made of glued laminated timber (glulam) products. But to deal with glulam we must also first be clear on what a 1 x 12 (or 2 x 6, or 2 x 4, etc.) really is. In the lumber industry these dimensions are `nominal’, which literally means `in name only’. Thus a 2 x 6 is not really 2 in. x 6 in.; it is actually 1.5 in. x 5.5 in. A single 1 x 12 board is actually ¾ in. x 11-1/4 in. A single 2 x 12 is 1-1/2 in. x 11-1/4 in. People who deal often with lumber products either have the actual dimensions (of nominal sizes) memorized, or, or have at their fingertips the information quickly available in tabular form.

And now for glulam. Glued laminated timbers are made from sawn lumber pieces (or laminations, or `lams’). For example, a 5-1/8 in. x 12 in. glulam will be manufactured using eight 2 x 6s. The actual dimensions of the 2 x 6s are 1.5 in. times 5.5 in. Eight of these `layed up’ (laminated) wide face to wide face gives a product that is 5.5 in. wide and 8 times 1.5 in. equals 12 in. deep. The 5.5 in. width is planed down to 5-1/8 in. The board feet measure of glulam is based on the input laminations; so, the 5-1/8 by 12 is 8 times 2 times 6 divided by 12 or 8 board feet (per foot of beam). If the beam is 18 ft long, it has a board foot measure of 8 times 18 equals 144 BF. Or, since we already said that a 2 x 6 is one BF per foot, and since there are 8 of them in the beam layup, we have 8 BF per foot of beam; 8 BF per foot times 18 ft gives 144 BF total.

Now let’s wrap this up with one more example. Let’s determine the board feet volume of a glulam beam that is 14-3/8 in. wide x 58.5 in. deep and is 72 feet long. The beam will be manufactured using 2 in. thick laminations that are actually 1.5 in. thick, and each lam (or layer) is made of a 2 x 6 and a 2 x 10 laid side by side (with the joints between the two staggered layer to layer). (This is referred to in the industry as a `composite’ beam in that each lamination or layer is `composed’ of more than one piece of lumber.) So, a single lam has 2 x 6 plus 2 x 10 equals 32 divided by 12 equals 2.667 board feet (per foot). There will be 58.5 divided by 1.5 equals 39 such laminations. Thus the whole beam will have 39 times 2.667 equals 104 board feet per foot. The entire 72-ft long beam will thus measure 72 times 104 giving 7488 BF. Whoa! … this beam has a board foot volume of over 7000 (board feet) … a 1 x 12 board 7000 ft (a mile and a half) long. Yeah, this is a big beam!

Suppose the beam is priced at $800.00 `a thousand’ (not a real price; just used for the example); this beam would be priced at $800 per thousand times 104/1000 thousand board feet per linear foot of beam equals $83.20 per linear feet of beam. The entire beam would be $83.20 per foot times 72 ft equals $5990.40. Alternately, the price of the entire beam could be calculated by 7488/1000 thousand board feet times $800.00 `per thousand’ giving (the same) $5990.40.

If we want some equations for this `Board Foot’ stuff,

BFPF = n x b x h / 12,

where

BFPF = Board Foot measure per foot of member (BF/ft),

n = number of input pieces (lams),

b = nominal width of piece(s) (in inches)

h = nominal depth of piece(s) (in inches).

Total BF = BFPF x L

where

L = length of member in feet.

For the 5-1/8 x 12 Glulam,

BFPF = 8 x 2 x 6 / 12 = 8.

The 5-1/8 x 12 glulam timber (itself) is considered to have a nominal (overall member section) dimension of 5 x 12 (or sometimes 6 x 12). A 5 x 12 sawn timber is a `single’ piece (of actual dimension 4.5 x 11.5). The 5 x 12 timber, if given BF measure, is thus,

BFPF = 5 x 12 / 12 = 5 … (way less than the similar size Glulam … interesting, or not).

References

Timber Construction Manual, 5th ed., American Institute of Timber Construction, 7012 S. Revere Parkway, Suite 140, Centennial, Colorado, 2005 (pages 3-5).