Unlike softwoods, which are sold in standardized dimensions, hardwoods are sold in random widths and lengths. So to tell how much wood hardwood boards contain, they are measured in board feet. “Board Feet” is a volume measurement. One board foot equals 144 cubic inches.
Often, a board foot is illustrated as a board 1"-thick x 12"-wide x 12"-long, but you’ll rarely find lumber in those dimensions. Any board containing a total of 144 cubic inches of wood equals one board feet, regardless of its proportions.
To calculate the number of board feet in a piece of lumber, multiply the board’s thickness by its width, then its length, all in inches. Then divide by 144:
(thickness x width x length)/144
For example, a 1″ × 9″ × 96″-long board equals six board feet (1×9×96=864/144=6). If this board were 1-1/2"-thick, it would contain 9 board feet (1.5×9×96=1296/144=9).
For these purposes, a 3/4"-thick board is considered to be a full inch thick.
Thinner stock is not typically described in board feet. Also, hardwood thickness is expressed differently than softwood — in quarters of an inch. A 4/4 (say “four-quarter”) board is 1"-thick. A 1-1/2"-thick board is expressed as 6/4. This often reflects the rough-sawn thickness. Surfaced lumber will be slightly thinner
I really had trouble teaching board feet to my wood working students. They had a hard time understanding that 1″ × 1″ × 144″ and 1″ × 12″ × 12″ were both board feet, even more so that any other combination that resulted in 144 cubic inches was board feet. Was something to see, though, when it sunk in.