Walnut lumber is sold by the board foot and is rounded to the nearest board foot. To calculate the material to order multiply the thickness x width x length. Then divide by 144 this is the amount of board feet in your lumber. For multiple boards add the board feet together and round to nearest whole number for purchasing.
Example calculation would be a board that is 1" thick x 7" wide x 60" long. Equals 420/144=2.9 Board feet.
2" thick x 7" wide x 60" long =840/144=5.8 board feet.
You would purchase 9 board feet.