Softwood is sold in standard dimensions, hardwoods is generally sold in random widths and lengths. So to tell how much hardwood is in a board, they are measured in board feet. A board Foot 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 rarely will you find lumber in those dimensions. Any board containing a total of 144 cubic inches of wood equals one board foot, regardless of its proportions.
Let's say you've found a board that's 4/4 - hardwood is measured in 1/4" (so 4/4" = 1") x 8" x 96"
To find the volume of wood, or the number of board feet, simply use this formula.
(TxWxL)/144 (units all in inches)
So our piece of wood 4/4" x 8" x 8'
Board feet = (1x8x96)/144 = 5.3 Bft.