What is a Board Foot?
If you are buying lumber from a big-box store, you usually pay by the linear foot (the total length of the board). However, if you are buying hardwood or rough-sawn lumber from a sawmill or specialty dealer, you pay by the board foot (BF).
The Formula
A board foot is a measure of volume. One board foot is equal to a piece of wood that is exactly 12 inches wide, 1 foot long, and 1 inch thick (144 cubic inches). The formula to calculate board feet is:
(Thickness in inches × Width in inches × Length in feet) ÷ 12
Quarters (4/4, 8/4, etc.)
Hardwood thickness is usually expressed in quarters of an inch. For example, "4/4" (pronounced "four-quarter") means the board is roughly 1 inch thick. "8/4" means the board is 2 inches thick. If you are buying 4/4 lumber, you can enter 1 into the thickness field.
Nominal vs. Actual
Keep in mind that if you are calculating standard construction lumber (like a 2x4), the actual dimensions are smaller (1.5" x 3.5") than the nominal name. However, in the hardwood industry, calculations are generally based on the nominal rough cut before planing.