All three dimensions—Thickness (T), Width (W), and Length (L)
The Formula:
Why 144?
A board foot is defined as 12" x 12" x 1".
If you multiply those dimensions, you get 144 cubic inches. By dividing your total cubic inches by 144, you are simply finding out how many of those "144-inch blocks" fit into your board.
Example:
Size of board: 17" x 6" x 1"
Material: Rustic Walnut
Price Per Board Foot: $8.00
17x6x1 = 102 cubic inches
102/144 = .708333333 board feet
.7083333333 * 8.00 = $5.67