How do you figure square footage from halfway point in the wall?

Question

I need to add 3" for walls. Actual room measurements are 12'4" x 14'4".

Answer 1

ADD 3" ???

Sq. ft. has forver been calculated as length x width; or length/width x height.

Without at all knowing your issue I suspect I might seem ignorant???

This is weird sorry. Are you speaking of something like X Ray vision or the wall being invisible and measuring into HALF of the enclosed wall to the opposite side also enclosed?????????????????????????

Your wall thickness, should be closer to 4 & 5/8 if drywall exists on bothe sides, so you might assume that half that is 2 3/8 PLUS/MINUS...... & YIKES,,,,,,,,,,,, this isn't my job.

No offense but this is Middle school math.