Calculate Board Feet? Go Figure.

Figuring out board feet is a way of life for me. Both logs and the lumber produced are figured in board feet, so I can’t really escape it, whether I am in the shop or at the sawmill. It is easy for me to forget that this isn’t an everyday occurrence for everyone else, even full-time woodworkers. I know that most woodworkers can explain what a board foot is, or at least won’t admit that they don’t know what it is, but I am still surprised when I ask someone how much lumber they need and they respond along the lines of, “Oh, six or seven boards.” This doesn’t tell me much because I don’t know how thick, long or wide those six or seven boards are in someone else’s mind. That’s where board feet come into play.

Board feet is a measure of the volume of wood, not just the surface area. A single board foot measures one square foot x 1″ thick. By the way, the thickness is based on the rough-cut thickness, not the finish-planed thickness. So, 4/4 thick hardwood lumber (rough-sawn at 1-1/8″ thick and finish-planed to 3/4″ thick) that has one square foot of surface is equal to one board foot. If that same square foot was 2″ thick, it would measure two board feet.

It would be easy enough to figure it out if all wood came in 12″ x 12″  1″ chunks, but it doesn’t. Random widths and random lengths are standard for hardwoods, which creates the need for some math and a better understanding of the calculations. Let’s start with just a single board. Here are three ways to approach it:

• Length (in inches) x Width (in inches) x Thickness (in inches) ÷ 144
• Length (in feet) x Width (in inches) x Thickness (in inches) ÷ 12
• Length (in feet) x Width (in feet) x Thickness (in inches)

Let’s plug in the dimensions of a specific board to see how it works. The measurements are Length (96″ or 8′)  x Width (9″ or .75′) x Thickness (1.5″)

• Length (96″) x Width (9″) x Thickness (1.5) = 1296 ÷ 144 = 9 bd. ft.
• Length (8′) x Width (9″) x Thickness (1.5) = 108 ÷ 12 = 9 bd. ft.
• Length (8′) x Width (.75′) x Thickness (1.5)  = 9 bd. ft.

When measuring a stack of boards it is time-consuming and sometimes impossible to measure each individual board, so averages and estimations come into play. When approaching a stack of wood, I start off by trying to get an average length. This is done by eye and is more accurate if the boards are closer in length. It is difficult to get a good average in a stack that ranges from 16′ to 4′. In that situation it is best to make a couple of groups if you can and then get an average. After you get an average length of the stack, measure the width of the stack. Be sure to subtract the air-spaces from the average width of the stack. Next, get the thickness of the boards. Take the time to make separate piles for different thicknesses. If the pile is a mixture of thicknesses an average can also be taken. The last bit of accounting is to determine the number of layers in the stack. Here’s how the calculations go for stacks of wood:

• Length (in inches) x Width (in inches) x Thickness (in inches)
x Layers ÷ 144
• Length (in feet) x Width (in inches) x Thickness (in inches)
x Layers ÷ 12
• Length (in feet) x Width (in feet) x Thickness (in inches)
x Layers

Here are some real dimensions to see how it plays out. The average measurements of the stack are Length (120″ or 10′)  x Width (48″ or 4′) x Thickness (1″) x 20 Layers:

• Length (120″) x Width (48″) x Thickness (1″)
x Layers (20) = 115,200 ÷ 144 = 800 bd. ft.
• Length (10′) x Width (48″) x Thickness (1″)
x Layers (20) = 9600 ÷ 12 = 800 bd. ft.
• Length (10′) x Width (4′) x Thickness (1″)
x Layers (20) = 800 bd. ft.

I normally calculate using inches divided by 144 for all the measurements because my random length lumber is very random. Lumber from large mills is usually cut to the nearest foot with packs consisting of only two lengths, 8′ and 9′, for example. In that case, using feet instead of inches is the simpler method.

The easiest to calculate is 12′ long, 1″ thick lumber. An 8″ wide board is 8 bd. ft., no calculation necessary. Just change inches to feet and go home.

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About wunderwoods

Hi! My name is Scott Wunder and I am the owner of WunderWoods Custom Woodworking. We build wine cellars, built-ins and furniture from local woods, here in St. Louis, MO. Recently, I finished a three-year term as the President of the St. Louis Woodworkers Guild, which had me writing a monthly article for our newsletter. I love to write, especially about wood, and found that I still had more to say. Every day I run into something wood related that I realize some of my customers don't know and this seems like a great forum for sharing what I have learned (instead of telling the same story to each person). The main thing to remember is that I try to keep it light and as my wife always reminds people that have just met me, "He is joking."

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