Siding Squares Calculator, Openings Deducted
Deducts your door and window openings from total wall area to find siding squares needed, with a waste allowance built into the total.
squares = ceil(1,268 ÷ 100 × 1.1) = 14
How it works
Siding is estimated by the square — 100 square feet of wall — the same unit roofers use. To find how many squares your house needs, start with the gross wall area, subtract the openings, and divide by 100.
The gross wall area is the perimeter of the house times the wall height. For a rectangular footprint the perimeter is twice the length plus twice the width, so a 40 by 30 foot house has a 140-foot perimeter; at 10 feet tall that is 1,400 square feet of wall before any deductions. Then you subtract the doors and windows, because you do not side over them. A standard entry door is about 21 square feet and an average window about 15, so a couple of doors and a handful of windows can knock off a hundred square feet or more.
What is left is the net wall area. Divide by 100 for squares, then add a waste allowance — 10 percent is typical, more for lap siding with a lot of corners and short runs where off-cuts pile up. The calculator rounds the result up so you order whole squares. Trim, corner posts, and starter strip are separate items priced by the linear foot; this tool sizes the field siding itself.
If your walls include gable triangles above the eave line, add their area separately — a gable adds half the base times the height of the triangle. For a first pass, the perimeter-times-height method gets you close.
gross wall = 140 ft perimeter × 10 ft = 1,400 sq ft
openings = 2 × 21 + 6 × 15 = 132 sq ft
net wall = 1,400 − 132 = 1,268 sq ft
squares = 1,268 ÷ 100 × 1.10 = 13.9 → round up to 14 squares