A board foot is 144 cubic inches of wood — one inch thick, twelve inches wide, one foot long. It is a volume, not an area, which is why a 2×6 holds twice the board feet of a 1×6 at the same length.
The formula and the catch
Thickness in inches times width in inches times length in feet, divided by twelve. An eight-foot 2×6 is 2 × 6 × 8 ÷ 12 = 8 board feet.
The catch is that lumber is sold by nominal size, not actual. A 2×6 actually measures 1.5 by 5.5 inches. Board foot pricing uses the nominal number, which is why the calculator above asks for it.
Hardwood uses quarters
Rough hardwood thickness is written in quarters of an inch. 4/4 is one inch, 8/4 is two inches, 5/4 is an inch and a quarter.
Rough lumber is also sold at rough dimensions and loses about a quarter inch when planed. Buying 4/4 for a finished three-quarter-inch part is normal; buying it for a finished one-inch part is not.
Add for defects and cutting
Rough hardwood comes with wane, checks and knots that get cut away. Twenty to thirty percent over your net requirement is a working allowance for furniture work.
That is on top of any waste from the cut list itself. A project needing 40 net board feet is a 50 to 55 board foot purchase.
Why softwood is not sold this way
Dimensional softwood at a home centre is priced by the piece, because the sizes are standardised and everybody knows what a stud is. Hardwood is priced by the board foot because it arrives in random widths and lengths from the mill.
That difference is not arbitrary. A hardwood supplier cannot stock every width, so boards come as they were sawn and you buy volume.
It also means the same nominal board can cost very different amounts at two suppliers depending on how they grade and measure, which is exactly what board feet let you compare.
Surfaced, rough, and what you are paying for
Rough lumber is measured at its full sawn dimension. Surfaced lumber — S2S, planed on two faces — is measured at the rough size it started at, not the size you receive.
So a board sold as 4/4 surfaced arrives at about 13/16 of an inch thick, and you pay for the full inch. That is normal and it is the same everywhere, but it surprises people the first time.
Plan your project around the finished thickness you will actually get, not the quarter designation on the price list.