Siding Calculator (Squares)
Enter the total length of the walls, the wall height to the eaves, any gables, and the area of windows and doors.
How it works
Walls = wall perimeter × height to the eaves. Gable height = (gable width ÷ 2) × rise ÷ 12. Each gable = ½ × width × height (or width × height × 0.75 with Kaycan's rake-waste method). Net = walls + gables + other − openings. Squares = net × (1 + allowance) ÷ 100, rounded up.
Worked example: Kaycan's example house: 40 × 30 ft, 9 ft to the eaves: walls 2 × (40 + 30) × 9 = 1,260 sq ft. Two 32 ft gables at 5/12 (6.7 ft high) × 0.75 ≈ 161 sq ft each. Total about 1,582 sq ft = 15.8, so 16 squares, without subtracting openings.
Assumptions
- Gable method: Kaycan's planning guide uses triangle area (½ base × height); its estimating guide multiplies width × height by 0.75 to allow for cutting waste along the rake, and in that worked example does not subtract openings or add waste. Choose one approach so waste is not counted twice.
- Waste default 10%, as Kaycan's planning guide says to add and CertainTeed's form suggests for large projects.
- Rectangular walls of one height. Measure stepped walls, bump-outs and dormers separately and add them as other areas.
- This is a quantity estimate only. Sheathing, water-resistive barrier and flashing requirements are set by local codes and the siding maker's instructions; check local codes.
Frequently asked questions
How do I calculate siding squares?
Add the wall areas and gable areas, subtract windows and doors, add about 10% for waste and divide by 100. One square of siding covers 100 sq ft (Kaycan).
How do I measure a gable for siding?
Measure the width at the base and the height at the center, then use ½ × width × height. Kaycan's estimating guide finds the height from the roof slope: a 32 ft gable at 5/12 rises 16 × 5/12 ≈ 6.7 ft.
Should I subtract windows and doors?
Kaycan's planning guide subtracts them and adds 10% for waste. Its estimating guide skips the deduction and the waste, letting the openings cover the waste. Use one approach, not both.
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Sources
Formulas and reference values are from these sources. Page content last updated 2026-10-19.
- Kaycan vinyl siding planning guide (walls + gables − openings, + 10%, ÷ 100)
- Kaycan vinyl siding estimating and installation guide (worked example house)
- CertainTeed siding estimating form (gable and wall formulas; 10% waste on large projects)
- NIST Guide to the SI (SP 811), Appendix B.9: conversion factors (foot = 0.3048 m, ton, pound)