Are slot port calculators based off total volume ?
 

I am trying to make a slot ported 3 cubic foot box. I want it tuned to 34hz. I am using a soundstream t4-12 and using the recommended box volume. It only gives me a port length for round tube.

Heres my question. Say i go to a slot port calculator and enter that i have a 3 cubic foot box tuned to 34 hz with a port of 13.5" tall and 3" wide (internal, as in 3" between the inside wall and inside of wood im adding for my slot port). Say it gives me a length of 30". Is the calculator telling me that a box with external dimensions that give me 3 cubic feet inside requires a port that is 13.5" tall, 3" wide and 30" long to be tuned to 30hz (thus the airspace for the sub is less than 3 cubic feet because the port takes up space), or is it telling me to take my existing 3 cubic feet and make the enclosure larger to add the port (in other words my external dimensions will not make an internal volume of 3 cubic feet, more like 3.3cubic feet or so) ? Thanks so much, i have tried searching but cant seem to find a good answer !

Most box recommendations, if not accompanied by a set of dimensions, are net volume so you must add for vent displacement.