How do you calculate gross area?

Measure the outside of the exterior walls, break the footprint into rectangles, multiply length by width for each one, and add them up. Do the same for every floor and add the floors together. That total is the gross area.

Measure to the nearest inch or tenth of a foot, work out each rectangle, and round only the final total. Odd shapes like bay windows or angled walls get their own small rectangles or triangles.

Gross living area starts from the same exterior numbers and then takes things out. Subtract the garage, unfinished rooms, the open space above a staircase or two-story foyer, and any area with ceilings below the ANSI height limits. Below-grade finished space is reported separately instead of added in. What is left is the figure an appraiser puts on the GLA line.