What is a board foot?
A board foot is a volume equal to a board 1 inch thick, 12 inches wide and 12 inches long. Hardwood dealers commonly quote lumber by board foot, while dimensional construction lumber is more often sold by the piece or linear length.
Why add waste?
Projects lose material to defects, trimming, grain selection, saw kerf and layout. A waste allowance is not universal: straight, repetitive parts may need little extra while furniture parts selected for appearance can require substantially more.