Estimate siding — square footage and number of squares — from your wall area minus openings, with a waste factor.
net = wall_area − openings; squares = net ÷ 100
Subtract window and door openings from the wall area, then add waste. Siding is often sold by the square (100 ft²).
Use 10% for simple walls; more for many corners, gables, or patterned coursing.
A 40 ft wide wall 10 ft tall is 400 square feet. Subtract two windows and a door, roughly 60 square feet, for 340 square feet net. Siding is sold by the square (100 sq ft), so that's 3.4 squares, and at 10% waste about 3.7 squares — round up to 4 squares ordered.
Gable walls add a triangle above the main wall; measure that separately (half the base times the height) and add it in. Corners, trim, and J-channel are additional linear-foot items not captured in the square count.
Siding, like roofing, is measured in squares of 100 ft². Subtract large openings but keep waste for cuts.
100 square feet — the standard unit for ordering siding and roofing.