If you draw a 10x10 box, it gives you 95 sf. Draw a line through it and it is less so it doesn't include interior walls. You have to take out for exterior walls too because it calculates it wrong. To find the exterior wall space of a 10x10 room, you must convert 3 in wall. divided by 12=.25 x 10=2.5, then times 4 is 10 sf. If it counted the exterior walls it would be 105 sf., not 95, is way off.