Siding Calculations
How to Calculate Siding: A Step-by-Step Guide
Published June 2, 2026
Calculating how much siding you need comes down to five steps: measure your walls, add your gables, subtract your openings, add a waste allowance, and convert the result to siding squares. Here’s how each step works.
Step 1: Measure every exterior wall
Measure the length and height of each wall you plan to side, in feet. Multiply length × height to get that wall’s area, then add up every wall’s area to get your total wall area.
A simple rectangular home with four walls — two at 28 ft long and two at 20 ft long, all 9 ft tall — has a total wall area of:
Don’t estimate this from your home’s listed square footage — floor area and exterior wall area are unrelated. Two homes with an identical floor plan can have very different wall area depending on ceiling height and roof design.
Step 2: Add your gable area
A gable is the triangular wall section under a peaked roof. Gable area is base × rise ÷ 2 — the standard formula for a triangle.
A gable with a 28 ft base and a 6 ft rise adds:
Add this to your wall area to get your gross area.
Step 3: Subtract windows, doors, and garage doors
Openings don’t need siding, so subtract their area from your gross area. For each opening, multiply quantity × width × height.
Six windows at 3 ft × 4 ft, one door at 3 ft × 6.67 ft, and one garage door at 9 ft × 7 ft together remove:
Subtracting this from gross area gives your net area — the actual siding coverage you need.
Step 4: Add a waste allowance
Waste accounts for cuts at corners and openings, angled gable cuts, damaged pieces, and pattern matching. A simple rectangular home commonly uses 10%; homes with more corners, dormers, or complex rooflines often need 15–20%.
Multiply your net area by (1 + waste %). At 10% waste on 793 sq ft of net area:
Step 5: Convert to siding squares
A siding square is a construction-industry unit equal to 100 sq ft, used to price and order material regardless of the specific product’s panel size. Divide your required area by 100:
Put it all together automatically
The Siding Calculator runs this exact process for you — enter your walls, gables, and openings, and it calculates net area, waste, required area, and squares instantly, along with a material-specific unit count and optional cost estimate. If you already know your material, use the vinyl, fiber cement, wood or metal siding calculator, or the siding material calculator to compare them.