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CALCULATING BOARD FEET, page 4
Converting square footage to board feet
Square footage is a surface measurement (SM) that does not consider whether something is 1" or 4" thick. In fact thickness is totally irrelevant. Think about carpet, ceramic tile or hardwood flooring. You always pay by the square foot regardless of a floor's thickness.
With rough lumber if you are in a situation where you must get involved in converting square footage to board feet, the math is pretty simple,
If all the wood is 1" thick in its rough state then square footage = board feet.
If you are buying 1 1/4" thick lumber then Board feet = square feet * 1.25 (bulked up for added thickness)
Thus in general the formula for board feet is square footage times the thickness of the wood (calculated in inches or parts there of).
Square feet * thickness of rough wood (inches) = board feet
Dimensional Lumber Sizes: 4/4, 5/4 or 8/4 ?
Maybe the is a good point in the conversation to introduce the concept of "quarters" . If you want to really talk about wood like someone who has been in the business for years, then you need to understand that 1" rough lumber is REALLY NOT 1" but 4/4.
For whatever reason, that I don't know, somewhere in our history we started referring to the thickness of lumber in 1/4" intervals. Thus 4/4 is in reference to (4) four - 1/4's . If you do the math:
1" thick material = 4 * 1/4" = 1" = 4/4 material
In some industries, they just want to drive you crazy. It's like trying to calculate board feet using the green thickness rather than just the width of the boards as they are right NOW! Note i didn't suggest any brains or logic behind this conversation, just a discussion of what is!
Thus, to carry the conversation further:
1 1/4" thick material = 5 * 1/4" = 5/4
1 1/2" thick material = 6 * 1/4" = 6/4
2" thick material = 8 * 1/4" = 8/4 NOT 2"
(note that 7/4 is pretty rare)
4" thick material = 16 * 1/4" = 16/4 (common in basswood)
So do you get the idea?
Thus when converting square footage to board feet it is important to know what thickness of material you are buying, NOT what thickness it is right NOW, in order to make the proper calculations.
NOTE: The conversion ALWAYS uses the "rough" thickness.
What is surface measure? (SM)
There are some situations, most notable in the craft market where wood often is milled in thicknesses of less than one inch, that they throw the concept of "board feet" out the window and start talking about square feet.
But even here the industry has to put their own twist on the conversation. They don't refer to it as square feet but rather "surface measure".
1 square foot = 1 unit of surface measure (SM)
You don't see this very often but if it comes up you'll know just to consider it as square footage. The end slab of wood that is left when you slice veneers is often sold on SM because they shave it all the way down to about 5/8" in thickness, and in random widths depending on what they cut from the log.
In conclusion:
Converting square footage to board feet is pretty simple as long as you understand what thickness you are really buying. If the retailer happens to have only 5/4 material and you really only need to plane the material down to 3/4" you are paying a 25% premium (board feet * 1.25* $$) for the privilege of producing more shavings.
Bottom line is you want to buy the thinnest board possible to still get the finished thickness required for the project.
Next I'll give you a few easy numbers on calculating linear feet to board feet, but if you would like to see how the industry calculates board feet with a scaling ruler or get a basic primer on lumber grades and how to figure out how much wood you need for your next woodworking project, sign up for this free wood working training on line.
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