Siding   1 Add surface areas of each side   sq. ft.   sq. ft.   sq. ft.   sq. ft.   sq. ft. SUBTOTAL sq. ft. 2 Add surface areas of dormer, gables, etc.   sq. ft.   sq. ft.   sq. ft.   sq. ft. SUBTOTAL sq. ft. 3 Add surface areas NOT TO BE COVERED. Windows sq. ft. Doors sq. ft. Chimneys sq. ft. Other sq. ft. SUBTOTAL sq. ft. 4 Add Step 1 and Step 2 together.   sq. ft. Subtract Step 3 from Total of Step 1 and Step 2   sq. ft. Add 10% sq. ft. 5 To determine number of squares needed, divide footage by 100.   sq. ft. Example: 1,282 sq. ft. Divided by 100 = 12.8 or 13 squares