Answer: Generally, one runs a tape measure around the outside walls of a home. Say its a rectangle of 30 by 50. 30 x 50 equals 1,500 square feet. Now, one can be a purest and subtract for wall thickness, from 4 to 6 inches. When measuring INSIDE rooms, its wall to wall. If a property ( or anything with sides) is not a square or rectangle, its complicated. Without referring back to geometry classes, one can separate the house into sections and add the sections together. This would be hard to understand without a diagram.