From WoodworkersZone WoodWiki
A standard measure of "hardwood" quantity. ("Plywood" is measured in square feet, and "dimensional wood" (construction lumber) is measured in linear feet.)
Equal to the amount of wood in a piece one foot square and one inch thick (144 sq/in.). The arithmetic formula used to calculate BF in a piece of hardwood is: (L x W x T)/144, where L=length, W=width, and T=thickness, all measured in inches. Note that thickness is typically calculated as "nominal" which is not the same as actual.
See also Buying Lumber