In article , John McCoy
wrote:

One of the local places measures to the 1/8" in width and to the
inch in length when figuring bd ft, and they don't allow much for
wane or splitting if present. Most everyone else figures to the
inch (rounding down) for width, and the half foot (rounding down
again) on length. They don't have to do it that way, but allowing
something for irregular boards is just being sensible. I don't go
to the first place much, it's not worth the aggravation. (btw,
this is all rough lumber...figuring to the exact half inch might
make more sense for S4S).

At the local place I buy from (there are really only two choices and
the other one doesn't stock as much variety or volume) they take out a
tape measure and a calculator. They may round to 1/4" on width, but if
they do it's down.