How this instrument works
A "square" is the standard ordering unit across the siding and roofing trades: 100 square feet of surface coverage, regardless of the panel size or profile that fills it. Contractors quote jobs in squares, suppliers price by the square, and cartons are packaged to cover a fraction of one — so converting a wall area into squares is the everyday first step of any siding estimate, long before anyone talks about panel counts or colors.
Waste allowance is the second half of the formula, and it is genuinely variable rather than one fixed number. A simple rectangular elevation with few openings loses relatively little material to cuts, so a 10% allowance — the figure commonly cited by siding manufacturers for straightforward jobs — usually covers it. A home with many corners, gables, dormers, or angled cuts wastes more material fitting panels around those features, and pushing the allowance to 15% or higher is standard practice for that kind of elevation. Treat the default here as a starting point to adjust for your own roofline, not a rule that applies equally to every house.
Net area is a direct input in this sheet rather than something recomputed from length, height, and openings — so the accuracy of the result depends entirely on the net area you enter. Measure the actual wall surface to be covered (gross wall area minus windows, doors, and any other openings) with a tape measure or from a marked-up elevation drawing, and feed that figure in directly.
- Measure or calculate your net wall area — gross wall area minus windows, doors, and other openings — in square feet.
- Enter that figure into the Net area field.
- Set a waste allowance: 10% for a simple rectangular elevation, 15% or more for a home with many corners, gables, or angled cuts.
- Read Squares needed, then round up to the next full or half square, since suppliers ship in fixed carton units.
Worked example — a 1,200 sq ft ranch elevation
A single-story ranch has 1,200 sq ft of net wall area once windows and doors are subtracted out. The footprint is a straightforward rectangle with only two corners per wall, so a 10% waste allowance is enough to cover ordinary trimming around openings and corners: squares = 1,200 × 1.10 / 100 = 13.2. The order rounds up to whatever partial-square increment the supplier packages — commonly the next half or full square.
Compare that to a smaller but more architecturally complex 800 sq ft cottage elevation with several dormers and inside corners, where a 15% allowance is more appropriate: squares = 800 × 1.15 / 100 = 9.2. Even though the second house has a third less wall area, its higher waste factor narrows the gap in squares ordered — a reminder that waste percentage, not just size, drives the final material count.
Questions
What does a "square" mean in siding?
One square equals 100 square feet of wall coverage — a trade-standard unit used across siding and roofing regardless of the specific panel profile. Suppliers price and package material by the square, so converting your wall area into squares is the number a contractor or supply house actually needs to quote or pull an order, rather than raw square footage.
How much waste allowance should I add for siding?
10% is the commonly cited default for a simple, mostly rectangular home with a normal number of window and door openings. Homes with more corners, gables, dormers, or angled cuts lose more material fitting panels around those features, so 15% or higher is standard practice for a more complex elevation. Treat any single percentage as a typical planning figure, not a guarantee — adjust upward if your roofline or wall layout is unusually intricate.
Does this calculator subtract windows and doors for me?
No — net area is a direct input here, not something the sheet recomputes from a gross wall size. Measure or calculate the actual surface to be sided (gross wall area minus windows, doors, and any other openings) yourself, then enter that net figure. Getting an accurate net area in is the single biggest factor in getting an accurate squares-needed result out.
Why do siding orders get rounded up to a full or half square?
Siding ships in cartons that cover a fixed, fractional portion of a square, and most suppliers do not sell true partial squares below that increment. Ordering the exact decimal result (say, 13.2 squares) would leave you short, so contractors round up to the next unit the supplier actually sells — commonly a half square — to make sure the job doesn't stall for lack of one more carton.
Is 10% waste always enough?
No. 10% is a reasonable default for a simple rectangular home, but it understates real material loss on elevations with many corners, gables, bay windows, or diagonal accents, where cutting around those features wastes proportionally more panel. For that kind of job, 15% or more is the figure experienced installers plan around — better to have a few extra pieces than to halt work waiting on a second delivery.