When you request a siding quote, contractors price the job largely from one number: the total square footage of exterior wall they need to cover. Understanding how that figure is built lets you compare bids fairly, spot an estimate that is padded or too thin, and plan a realistic budget before anyone visits your home.
This guide breaks down the exterior wall area formula, how door and window openings are subtracted, how gable ends add triangular area, and how the industry converts square footage into the "squares" used to order material. We will run a full example on a typical single-story home so every step is concrete.
The core formula: perimeter times height
Siding covers the vertical faces of your home, so the starting point is the total area of those walls. For a straightforward rectangular house, that area is the perimeter of the home multiplied by the wall height: perimeter × height. The perimeter is the sum of all the outside wall lengths as you walk around the building.
For example, a home with a 120-foot perimeter and 10-foot-tall walls has a gross wall area of 120 × 10 = 1,200 square feet. Measure the perimeter at ground level and the height from the foundation to the eaves. If the house has wings or bump-outs, walk the full outline so every jog is included rather than measuring a simple bounding box.
Subtract the big openings
Your gross wall area includes doors and windows, but you do not side over them, so those openings are subtracted from the total. Contractors typically use standard allowances rather than measuring every unit individually on a first pass.
A common rule of thumb is about 21 square feet for a standard exterior door and about 15 square feet for an average window. If our example home has two doors and four windows, that is (2 × 21) + (4 × 15) = 42 + 60 = 102 square feet to subtract, bringing 1,200 down to roughly 1,098 square feet. Only subtract large openings; small gaps are absorbed by the waste allowance rather than deducted.
- Standard exterior door: subtract about 21 square feet each.
- Average window: subtract about 15 square feet each.
- Garage doors: measure directly, since sizes vary widely.
- Leave small openings in the total — they are covered by waste.
Add the gable triangles
Homes with pitched roofs have gable ends — the triangular sections of wall that rise to the roof peak above the main wall height. These get siding too, so their area must be added back in. The area of each gable is a triangle: ½ × base × height, where the base is the wall width and the height is the vertical rise from the eaves to the peak.
A gable that is 30 feet wide at the base and rises 8 feet to the peak adds ½ × 30 × 8 = 120 square feet. A typical gabled house has two such ends, so gables can add a few hundred square feet to a project. Do not overlook them — leaving gables out is one of the most common reasons a DIY estimate falls short of a contractor's.
Add a waste allowance
Siding is cut to fit around corners, openings, and rooflines, and those cuts produce offcuts you cannot always reuse. To account for this, add roughly 10 percent to your net wall area. More complex homes with many corners, dormers, or steep roof angles may warrant 15 percent.
For the example home, suppose the net wall area after subtracting openings and adding modest gables lands near 1,073 square feet; adding 10 percent brings the order to about 1,180 square feet. The waste factor is not padding — it is the material that ends up in offcuts, and skipping it almost guarantees a shortfall mid-project.
Think in "squares"
The siding industry measures and sells material in units called squares, where one square equals 100 square feet of coverage. This shorthand keeps ordering and labor pricing simple: a job is quoted as a number of squares rather than raw square feet.
To convert, divide your total square footage by 100. Our example's roughly 1,180 square feet is about 11.8 squares, which rounds up to 12 squares of siding. When you see a quote priced per square, you can now sanity-check it: multiply the price per square by the number of squares and compare it against the total.
Order trim, corners, and starter separately
Field siding is only part of the material list. The finishing pieces — outside and inside corner posts, J-channel around windows and doors, starter strip along the bottom course, and fascia or soffit trim — are ordered by linear foot, not by square footage, and they are priced separately.
Because these accessories follow edges and openings, estimate them by measuring linear feet: total the vertical corners for corner posts, the perimeter of every window and door for J-channel, and the base of each wall for starter strip. A quote that lists only squares of field siding is incomplete; a thorough estimate itemizes trim so you are not surprised by add-ons later.
Worked example: a 120-foot perimeter home
Bringing it together for a single-story home with a 120-foot perimeter and 10-foot walls: gross wall area is 120 × 10 = 1,200 square feet. Subtract two doors and four windows (about 102 square feet) to reach roughly 1,098 square feet, then add modest gable triangles and trim the net down through openings to land near 1,073 square feet of siding surface.
Adding a 10 percent waste allowance brings the order to about 1,180 square feet, or roughly 12 squares. Add itemized linear-foot quantities for corners, J-channel, and starter strip, and you have a complete picture you can hold any contractor's quote against.