I am fairly new to calculating wood. How would I calculate the amount of board feet I need to purchase?
Say I am making a face for a floating shelf. 3/4″ X 2 1/2″ X 6′
Do I literally just punch those numbers in and get the answer? Or do I figure out what raw stock size I need and work from there?
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Except for dealing with dull tools, lumberyard lingo is one of the most frustrating things when getting started in woodworking. The important thing is that you find the best board for the project. So, go digging through the stack of boards, find what you think you'll be able to use best and then worry about paying for it.
You really only need to worry about board foot calculations when you're paying for lumber at the lumberyard. Since most lumberyards sell boards at random widths, they can't sell using a per-board price, or a linear price like big box stores. Same way you pay for deli meat by the pound, you pay for lumber by the board foot. It's the lumberyard's way of calculating the actual amount of wood you're buying.
Here is a quote from Steve Scott's article "The Language of the Lumberyard" that explains it well.
https://www.finewoodworking.com/2011/07/28/the-language-of-the-lumberyard
"Lumberyard operators say one of the biggest challenges new customers face is in understanding the board foot—the basic unit of measurement for roughsawn stock. The board foot (144 cubic in.) is confusing because it measures a board’s volume, not its length. This means that a piece of stock 1 ft. long can contain more than 1 board foot of material. A good visual way to understand 1 board foot is to picture a board 1 in. thick by 12 in. wide and 12 in long. Add an inch to the board’s thickness, and you now have 2 board feet. To calculate a plank’s board footage, multiply its thickness by its length and width (all in inches) and divide the result by 144. In contrast, surfaced lumber is typically sold by the linear foot, a simple measurement of a board’s length. The price per foot will vary according to the board’s width and thickness."
As described previously it is simply a measure of volume. Using ft. for the length and dividing by 12 is simpler and more in line with how we think of lumber. Thickness (in) X Width (in) X Length (ft) divided by 12
1" X 12" X 12' = 12bf
1" X 6" X 12' = 6bf
2" X 6" X 12' = 12bf
You would also have to consider the nominal size of the board your finished piece comes from. Saw mills generally cut the following thicknesses, referred to as rough lumber:
4/4, 5/4, 6/4, 8/4, 10/4, 12/4, 16/4. "Four Quarter" = 1" thick, "Eight Quarter" = 2" and so forth.
So to determine the bdft needed for your face piece, it would have to come out of 4/4 lumber and the calculation would look like this:
1" X 2.5" X 6' divided by 12 = 1.25bf
If you are shopping at other than a BORG, take your measurements and buy a board that fits your dimensions and avoids any defects. You are unlikely to find a board 2 1/2" wide, and somewhat unlikely to buy one 6' long. You will probably end up with a board 5-6" wide and 8' long. Pick your board and pay the man what the BF measure is for your choice.
If you are doing a larger project, say a table, figure out what you need an allow for 20% or so for cutoff and ripping waste.
All good info so far Larue. I will add some more based on things you will run into beyond the raw calculations. As mentioned, if you are buying your material at a BORG (Big Ole Retail Giant) you can pick out pre-dimensioned boards that will yield the parts you need. That is, 1 board foot of lumber that is 1" x 12" x 12" will not meet your needs if you need a board 2" wide and 34" long. You would pick a board that you could cut the 2" wide, 34" long part out of.
Since the board foot is a measure of volume it does not translate well to a parts list (or cut list). It is the unit that lumber yards use to charge for material. This means if you are trying to calculate price for a project, you need to understand how the volume unit of a board foot translates to material in hand that will yield the parts you need.
This requires a paradigm shift in thinking from the BORG shopping experience. A possible difference is in selecting a volume of material (a board) that will yield four table legs versus buying a BORG board of the proper width and height dimensions that is long enough to yield four table legs.
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