You’ll use board footage when you purchase lumber from a commercial or hardwood lumber yard (instead of per piece pricing), and there are good reasons for it.
When you buy construction-type lumber from the home improvement center, almost always you’ll be buying pieces of lumber that are standard construction sizes, commonly known as “1 by”, “2 by” and “4 by” lumber.
Because there is only a limited amount of these common construction lumber sizes, this works well and is easy to calculate. You simply multiply the piece price by the number of pieces you need, and you know what you’ll be spending.
So, if this works so well, why mess with it? And, WHAT THE HECK IS A BOARD FOOT?
The answer to the first question is “necessity.” A different measurement and pricing system is needed in hardwood and partially dimensioned lumber sales. For one thing, there is almost an infinite amount of sizes in the board bins. Sure, the lengths are familiar. The hardwood dealer we go to has most of the bins filled with 12’ lumber. You can buy a partial length, but at least 4’ must be left “for the next guy.”
However, depending on where the lumber was cut from in the log, how thick it was cut, and how wide a board resulted, there are hundreds of sizes of boards. That’s where board feet come in.
The answer to the second question is that a board foot is a unit of measurement for the VOLUME of wood in a given board. A basic board foot is defined as 12” x 12” x 1”. Multiply these three numbers, and you get 144. 144 what? Cubic inches, that’s what.
There are TWO ways to formally calculate the amount of board footage in any piece of lumber.
If the length of the board is in even feet or easy fractions thereof (halves and quarters), then you use this formula: (length in feet X width in inches X thickness in inches) / 12. Let’s use the example of a board that is 9” wide, 1-½” thick, and 10-½ feet long. Our calculation would be 9 X 1.5 X 10.5. This equals 141.75. Divide this by 12 and you see that you have about 11.81 board feet.
If the board is in an odd length, use a slightly different, but related, method. Use INCHES for the length and divide by 144. The formula will now look like this: (length in inches X width in inches X thickness in inches) / 144. Let’s use the example of a board that is 10” wide, 2” thick, and 140” long. Our calculation would be 10 X 2 X 140. This equals 2800. Divide this by 144 and you see that you have about 19.44 board feet.
One last thing. Hardwood lumber is sold in thicknesses expressed in quarters - when the lumber is over ¾” thick. 1” thick is “4-quarter”, 1-¼” is “5-quarter”, 1-½” is “6-quarter”, then you jump to 2” (8-quarter) and then 3” (12-quarter). This is the way the sizes are said, but they read as 4/4, 5/4, 6/4, 8/4 and 12/4.
Also, these are the names of the lumber, but they are approximations of the thickness, as when the lumber is face dressed, it will end up being slightly thinner.
There you have it! Now you can hold a well-informed conversation with your lumber supplier and calculate and communicate confidently.