Siding Calculator

Enter your wall perimeter, height, and total door/window opening area below to calculate how many squares of siding your project needs. Siding, like roofing, is bought and priced by the square (100 sq ft of coverage), but because that per-square pricing rewards precision, this calculator subtracts your actual openings before converting to squares, rather than skipping that step.

Wall perimeter (ft)

Wall height (ft)

Door/window openings (sq ft)

Waste (%)

Gross wall area

360 sq ft

Net wall area

360 sq ft

Squares needed

3.60 sq

Squares to order

4 sq

Net wall area already has door and window openings subtracted. Squares to order includes a 10% waste factor and rounds up to the next whole square. Suppliers don't sell partial squares.

Already familiar with “squares” from our roofing calculator? Same unit, same 100-square-foot definition: this page just applies it to exterior walls instead of roof planes.

The “Square” Isn’t Just for Roofs

Siding is bought and priced the same way roofing is: by the square, equal to 100 square feet of coverage. Suppliers quote siding jobs in squares, and installers bid labor per square rather than per linear foot of board or per individual panel. If you’ve used our roofing calculator before, this is the identical unit. The only difference is which surface of the house you’re covering. A roofing square covers 100 sq ft of sloped roof plane. A siding square covers 100 sq ft of vertical wall.

Where Siding Estimating Splits From Drywall Estimating

This is worth calling out directly, since it runs opposite to advice elsewhere on this site. On our drywall calculator, we recommend skipping the deduction for ordinary-sized doors and windows, because drywall is bought in whole 4×8 sheets. A small cutout rarely shaves off enough area to change your sheet count once you round up, so the extra measuring isn’t worth the effort.

Siding works differently. Because it’s priced and ordered by the square, a coarser and higher-cost unit covering 100 sq ft at a time, the combined area of several doors and windows on a typical wall run adds up to a meaningful fraction of a square, not a rounding error. Skipping the deduction on a siding order means paying for material you’ll never install. So unlike drywall, siding math subtracts openings before converting to squares.

From Wall Area to Order Quantity

Gross area = Perimeter ft × Height ft. Siding wraps the exterior, so the natural input is total wall perimeter rather than a length-times-width footprint the way a room or roof deck is measured.

Net area = Gross area − Total opening area. Subtract your combined door and window area per the point above.

Squares (with waste) = (Net area ÷ 100) × (1 + waste %), rounded up to the next whole square before ordering. Like roofing, suppliers don’t sell partial squares.

Wall Complexity	Typical Waste Factor
Simple rectangular walls, few corners	10%
Walls with multiple corners or gable ends	12–15%
Complex walls: dormers, cutouts, accent trim	15–20%

Example: A house wall run with a 40 ft total perimeter and 9 ft wall height, 60 sq ft of combined door and window openings, and 10% waste.

Gross area: 40 × 9 = 360 sq ft Net area: 360 − 60 = 300 sq ft Squares: 300 ÷ 100 = 3.0 → with 10% waste: 3.0 × 1.10 = 3.3 → round up to 4 squares

Notice why the opening deduction matters: skip it here and the gross 360 sq ft carries all the way to 3.96 squares with waste, which happens to round up to the same 4 squares in this particular example. But on a longer wall run with proportionally more window and door area, skipping the deduction can just as easily push your order a full square higher than it needs to be. The difference isn’t always this forgiving.

The 10 to 20 percent band above is a siding convention rather than a universal one. Waste allowances differ by trade, and for some materials the margin is covering compaction or settling rather than offcuts, which behaves quite differently when you scale the job up. The material waste factors guide puts the figures side by side.

Common Siding Materials

• Vinyl siding: the most common re-siding material; lightweight, inexpensive per square, and typically the lowest waste factor of the group.
• Fiber cement lap siding: durable and fire-resistant, but heavier and more prone to cracking during cuts, which usually pushes waste toward the higher end of the range.
• Wood or engineered wood lap siding: natural material variation and finishing prep mean planning slightly more waste than vinyl.
• Shingles and shakes: smaller individual pieces mean more cuts around openings and corners, usually landing at the top of the waste range.

Fiber cement, wood and engineered wood all get painted, and none of them takes paint at the rate printed on the can. For coverage on a porous or textured exterior, and how many gallons that actually works out to, see the paint coverage rates guide.

Gable Ends Are Where the Waste Really Comes From

A wall is a rectangle and estimates cleanly. A gable is a triangle, and it is the single biggest source of siding waste on a typical house.

Gable area = (Width × Height) ÷ 2, where height is the vertical rise from the top of the wall to the ridge, not the rafter length. If you know the pitch but not the rise, the roof pitch calculator converts one to the other.

The area is the easy part. The waste is not. Every course running into a gable meets the rake at an angle, so every piece needs an angled cut, and the offcut from one side only occasionally fits the other. On a house with two gables, this is usually what pushes a 10% allowance to 15%.

Courses, not square feet, are what you install. Siding is hung to an exposure, the visible height of each course once the next one laps over it, and wall height divided by exposure gives the course count. It rarely divides evenly, which means deciding in advance whether the odd course lands at the top, where it is visible against the soffit, or at the bottom, where the starter strip hides it.

Openings follow the same rule as elsewhere on this site: subtract large ones, ignore small ones. A window under about 10 square feet generates offcuts you will not reuse, so deducting it produces a number that is precise and wrong.