sub displacement

Check the manufacturer's specs.  I read that a standard for 12's was .07 but today's subs are .85 and larger.  What's the sub, Ketel, or are you working on a hypothetical setup?

The manufacturer of the sub should supply what the recommended air space is.  Once you know what the recommended space is, you can find it by mutiplying the width, length, and the height of the internal dimensions.Rhino - Bringing you only the best in car audio!!

i was looking for a kappa perfect 12.1dvc for a friend who wants to build his own box.Quad L Handyman services

81.94 square inches for that particular sub

square inches??  If that was a typo and meant cubed, it would equal .047cu.ft.  That's a small 10.  Like I said, a 12 is .07, .085 cubic feet.  Check the manufacturer for exact displacement.  They most likely find out displacement  the easiest and most accurate way...that is, actually displace a liquid instead of air and then measure the remaining liquid after the speaker was set in place.

My bad, I misread I thought you were asking for the drivers cone area.  Not sure what the hell I was thinking.