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Siding Calculator

Squares of siding for a house, with gables and openings.

Your measurementsEvery assumption editable01

Half the gable width times its height, per gable. Set to 0 for a flat-fronted wall.

Each door is treated as 21 sq ft.

Each window is treated as 15 sq ft.

10% for a simple rectangle, 15% for a house with many corners and openings.

Siding requiredCalculated live02

Result

12.08squares

12.08 squares — 1207.8 sq ft

Wall area140 ft perimeter × 9 ft high
1260 sq ft
Deducted for openings2 doors, 8 windows
−162 sq ft
Net area to cover
1098 sq ft
With waste allowance+10%
1207.8 sq ft
Siding squaresone square covers 100 sq ft
12.08

Siding is quoted and sold by the square, the same hundred-square-foot unit as roofing. Boxes per square vary by product and profile, so convert at the merchant.

Trim, corner posts, J-channel, starter strip and fasteners are all separate and are priced by the linear foot. On a house with many windows the trim can rival the siding in cost.

Only deduct openings that are genuinely large. Deducting every small window tends to leave you short, because the offcuts around an opening are rarely reusable elsewhere.

Enter the building perimeter and wall height, add any gable area, and deduct the doors and windows. The result is in squares — the hundred-square-foot unit siding is quoted and sold in, the same as roofing.

Why use this tool?What it does differently03

Why use this tool?

Gables handled separately

Gable ends are triangles and easy to forget. A dedicated field keeps them in the total.

Perimeter, not four walls

One measurement around the building replaces measuring each elevation separately.

Answers in squares

The unit suppliers quote in. Boxes per square vary by profile, so convert at the merchant.

Sensible deductions

Doors and windows come off at standard sizes, with guidance on which openings are worth deducting at all.

How this worksThe method04

How this siding calculator works

Wall area is perimeter times height, with gable area added on top. Doors and windows are deducted at 21 and 15 square feet respectively, the waste allowance is applied, and the result is divided by 100 to give squares.

Gable area for a simple triangle is half the width times the height of the triangle. Add the figures for every gable before entering the total — a house with two gable ends has two to include.

Only deduct openings that are genuinely large. Subtracting every small window tends to leave you short, because the offcuts from around an opening are usually too short to use anywhere else.

How to use itStep by step05

How to use it

  1. Step 1: Measure the perimeter

    Walk around the building measuring at ground level and add the wall lengths together.

  2. Step 2: Enter the wall height

    Ground to eaves. Gable triangles above that line go in the separate field.

  3. Step 3: Calculate the gables

    Half the gable width times its height, per gable, added together.

  4. Step 4: Count the openings

    Include doors and windows. Skip anything small enough that the surrounding offcuts would be unusable.

Example usageWorked figures06

Example usage

A simple two-storey box
A 140 foot perimeter at 9 feet is 1,260 square feet. Less 2 doors and 8 windows at 162 square feet, plus 10% waste, gives 1,208 square feet — about 12.1 squares.
Adding gable ends
Two gables 30 feet wide and 6 feet high add 180 square feet, taking the same house from 12.1 to 14.1 squares. Gables are routinely forgotten and routinely cause a second order.
A house with many windows
The same perimeter with 16 windows deducts 240 square feet rather than 120 — but raise the waste allowance to 15%, because every opening is a run of cuts.
Frequently asked questionsCommon questions07

Frequently asked questions

How much siding do I need?

Wall area plus gables, less large openings, plus 10 to 15 per cent waste, divided by 100 for squares. A typical two-storey house runs 12 to 20 squares depending on footprint and gables.

What is a square of siding?

One hundred square feet of coverage — the same unit used for roofing. Boxes per square vary by product and profile, so convert from squares to boxes at the supplier.

How do I calculate gable area?

Half the width of the gable times its vertical height, for each gable. A 30 foot wide gable rising 6 feet is 90 square feet, and most houses have two.

Should I deduct every window?

No. Deduct doors and large windows, but leave small openings in the total. The offcuts from around a small window are rarely long enough to reuse, so deducting them tends to leave you short.

Is trim included?

No. Corner posts, J-channel, starter strip, soffit and fasteners are all separate and priced by the linear foot. On a house with many windows the trim can approach the cost of the siding itself.

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