How to calculate the Perimeter of a Floorplan. Just add up the individual sides. You must take care not to count any side twice. Your browser does not support the canvas element. 28 + 12 + 20 + 20 + 20 + 15 + 20 + 25 = 160 ft 20 + 12 = 32ft Total = 160 + 32 = 192 ft.