Board feet uses nominal softwood and rough hardwood size, not the finished size

What this diagram shows

A comparison of how board feet is figured for softwood and hardwood. On the left, a 2 by 4 end-section: the dashed outline is the nominal 2 inch by 4 inch size and the solid wood inside is the actual surfaced 1½ inch by 3½ inch. Board feet is figured on the named 2 by 4, so a 2×4×8 is 5.33 board feet — the value you buy and pay for — not the 3.5 board feet you would get from the smaller surfaced size. On the right, the hardwood quarter system: rough thickness bars for 4/4 equal to 1 inch, 5/4 equal to 1¼ inch, 6/4 equal to 1½ inch, and 8/4 equal to 2 inches. Hardwood is sold rough by the quarter-inch; a 4/4 board is 1 inch rough and surfaces to about 13/16 inch, but board feet and price use the 1 inch rough size.