dividers in a subwoofer box

 I figured out my trunk measurements and the required air space for the speakers.I've used the 12 volt calculator and it tell me to subtract the thickness of the dividers from the internal air space of the box.I don't understand what to do to get the correct air space.The speakers I'm using require 1.5 cubic ft of air space.My measurements are as follows ; width = 31.5 ; depth = 30 ; height = 14

Assuming that you are using 3/4 inch material, the dimensions you posted has an internal volume of 6.185 cubic feet.  How many woofers do you have?  Which dimension were you planning to mount the woofers to?  What size are the woofers?  Which of the dimensions would you like to reduce to achieve the correct airspace? 

Are you planning on having all 4 woofers mounted on the 30 x 31.5 inch piece?   What is the outside dimension of the drivers?

The mounting depth is 8 5/8 in. and the cut out is 13 3/4 and yes i hope to have all 4 mounted.

 I was trying to look on the Kicker website,but it don't tell me the outside dimension.I haven't bought the speaker yet.I'm trying to see if they will fit with those measurements.

Without a Crystal Ball, or the outside dimension of the driver, I can not help you.  If they will fit it is going to be tight.

1.5 cubic feet appears to be the minimum volume for a sealed enclosure.   You would probably be better off with 2 drivers in that box.

The square cut out is 13 3/4