Siding calculator: squares for walls and gables
Siding is sold in squares of 100 sq ft. Add up each wall's width times height, add half the base times the height of every gable, subtract windows, doors and garage doors, then add 10% for waste. A 40 x 28 ft two-story house with two gables needs about 25.5 squares.
Updated · By the Interior Redesign AI editorial team · Sources cited on the page
Siding to order
25.5 squares
2,543 sq ft including 10% waste; 1 square = 100 sq ft (9.3 m²).
Net wall area
2,312 sq ft
walls + gables - openings
Whole squares
26
if sold by the square
| Walls (perimeter x height) | 2,448 sq ft |
|---|---|
| Gables (2 x 1/2 x base x height) | +196 sq ft |
| Windows, doors, garage doors | -332 sq ft |
| Net area | 2,312 sq ft |
| Waste (10%) | +231 sq ft |
| Squares to order | 25.5 |
Planning estimate. 10% waste is the allowance in Kaycan's estimator and CertainTeed's guide; opening sizes are editable assumptions. Trim, starter, J-channel, corners and soffit are not included.
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Key numbers
- 1 square
- 100 sq ft of wall covered
- Wall area
- Width x height of each wall, added up
- Gable area
- 1/2 x base width x height at the center
- Openings
- Subtract windows, doors and garage doors (Kaycan's method)
- Waste allowance
- 10% (Kaycan; CertainTeed for large projects)
- Gable height from pitch
- Half the house width x rise / 12
Worked example
A 40 x 28 ft two-story house
18 ft eave walls, two gables on a 6/12 roof, 12 windows (3 x 5 ft), 2 entry doors (36 x 80 in) and one 16 x 7 ft garage door, 10% waste.
- Walls: 136 ft perimeter x 18 ft = 2,448 sq ft.
- Gables: 14 ft x 6/12 = 7 ft high; 2 x 1/2 x 28 x 7 = 196 sq ft.
- Openings: 180 + 40 + 112 = 332 sq ft off.
- Net: 2,644 - 332 = 2,312 sq ft; plus 10% = 2,543.2 sq ft.
Order about 25.5 squares of siding (26 if it is sold in whole squares).
How we calculate
We treat the house as a rectangle: the perimeter times the eave wall height gives the wall area. Each gable is a triangle across the house width, with its height set by the roof pitch: half the width times the rise per 12. Kaycan's estimator and CertainTeed's guide both measure walls and gables this way.
Openings come off the total, as Kaycan's estimator does. The default sizes (3 x 5 ft windows, 36 x 80 in doors, a 16 x 7 ft garage door) are assumptions for a quick estimate; type in your own sizes for a closer number.
Then we add the waste allowance, 10% by default, the figure Kaycan adds and CertainTeed suggests for large projects. Gables, lower roof pitches and fiber cement cut more, so you can raise it. CertainTeed's alternative is to add 1 ft to each gable's center height instead.
The result is siding only. Starter strip, J-channel, corner posts and trim are counted by length from the same walls, and soffit by area; your supplier will take those off your elevations.
Estimates, sources
Planning estimates and published guidelines, not quotes, designs or code rulings. Figures were checked on each source on the date shown. Local prices, your home and your jurisdiction's adopted code decide the real answer.
- Vinyl siding estimator (Kaycan, as of Oct 26, 2026): rectangular walls plus triangular gables, windows and doors subtracted from the total surface area, add 10% for waste, 1 square = 100 sq ft
- Estimating and installing vinyl siding (CertainTeed, as of Oct 26, 2026): walls: height x width per wall; gables: height at center x 1/2 width (add 1 ft to the height to allow for waste); for large projects add a waste allowance of 10 percent to siding, soffit and accessories; lower-pitch roofs produce more gable waste
Gable area by roof pitch
The steeper the roof, the taller and bigger each gable. For a 28 ft wide house, one gable covers:
| Roof pitch | Gable height | Gable area |
|---|---|---|
| 4/12 | 4.7 ft | 65 sq ft |
| 6/12 | 7 ft | 98 sq ft |
| 8/12 | 9.3 ft | 131 sq ft |
| 10/12 | 11.7 ft | 163 sq ft |
| 12/12 | 14 ft | 196 sq ft |
How much siding for a ranch house?
A one-story 50 x 30 ft ranch with 9 ft walls, two 5/12 gables, 10 windows, 2 doors and a double garage door works out to 1,326 sq ft of net wall, or about 14.6 squares with 10% waste. Single-story houses need far less siding than their floor area suggests, because the wall height is short.
Rake length matters for trim: at 5/12, each rake on that ranch runs 16.3 ft from eave to ridge.
How to use this tool
Measure the house
Length and width of the footprint, and the wall height from the foundation to the soffit.
Add the gables
Count the gable ends and enter the roof pitch (rise per 12 in).
Count the openings
Windows, doors and garage doors, with their sizes if they differ from the defaults.
Read the squares
Net wall area, the waste allowance and the squares to order.
Questions
How many square feet is a square of siding?
100 sq ft. A wall area of 2,312 sq ft is 23.1 squares before waste.
How much waste should I add for siding?
About 10%. Kaycan's estimator adds 10%, and CertainTeed suggests 10% for large projects. Houses with many gables, dormers or a low roof pitch waste more.
Do you subtract windows and doors when estimating siding?
Kaycan's estimator subtracts them. CertainTeed's quick method measures walls including openings, which builds in extra. Subtracting gives the tighter number; the waste allowance covers the cuts around them.
How do I calculate the area of a gable?
Half the base width times the height at the center. A 28 ft wide gable on a 6/12 roof is 7 ft high, so 98 sq ft.
How many squares of siding for a 2,000 sq ft house?
It depends on wall height and shape, not floor area. A 40 x 28 ft two-story house (2,240 sq ft of floor) needs about 25.5 squares; a 2,000 sq ft ranch needs a similar or smaller amount because its walls are short.
Does this include soffit, corners and J-channel?
No. It counts siding squares. Trim pieces are counted by length (starter along the bottom, J-channel around openings, a corner post at each corner), and soffit by area.
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