See the calculation with sample numbers
A 12 × 10 ft rectangle plus an 8 × 5 ft rectangle is 160 sq ft gross. Subtracting a 3 × 2 ft cutout leaves 154 sq ft.
What can change the result?
Perimeters are for each separate rectangle. The tool does not infer a combined outline for connected or irregular shapes.
How this was calculated: Gross/net square feet, outside perimeter of each rectangular area, room-by-room breakdown.