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.
Beginners size the rectangular wall faces and stop, but gable ends are triangles that add real area. A gable's area is half its base width times its peak height, measured vertically from the top of the wall to the ridge — not along the roof slope. Add every gable to the rectangular wall total before converting to squares.
Siding itself is only part of the order. Measure starter strip along the bottom of each wall (about the building perimeter), J-channel around every opening and roofline, and corner posts at wall height times the number of corners. Behind it all goes house wrap, sold by the roll — divide wall area by roll coverage and allow for seam overlap.
100 square feet — the standard unit for ordering siding and roofing.
A square is 100 square feet of wall coverage, the same unit used in roofing. Vinyl siding typically ships in boxes covering about two squares (200 sq ft).
Deduct only large openings such as picture windows and garage doors. Standard windows and entry doors are usually left in, since siding is cut around them and the offcuts are scrap that the waste factor covers.
About 10% for simple walls with lap or vinyl siding, rising to 15% or more for cut-up walls with many dormers and corners, or for diagonal and board-and-batten layouts.