Siding Calculator

What is a siding calculator?

A siding calculator tells you how much exterior siding you need to cover a wall, measured in squares — the unit siding is bought and priced by. One square equals 100 square feet (about 9.2903 m²) of finished wall coverage. You enter the wall’s length and height, subtract the area of any doors and windows, add a waste allowance for cuts and trimming, and the calculator returns the net area and the number of squares to order.

Working in squares keeps your estimate in the same units as the supplier’s quote, so you can price the job, order a single delivery, and avoid both running short and paying for surplus panels.

How does it work?

First, the calculator finds the gross wall area. A rectangular wall that is $L$ long and $H$ high has an area of:

Next it subtracts the total area of openings (doors and windows) and adds a waste allowance $w$ (a percentage) for cuts around corners, trim, and breakage, giving the net siding area:

Finally, because siding is sold by the square (100 ft²), it divides the net area in square feet by 100 and rounds up to the next whole square — you cannot order a fraction of a square:

The net area result can be shown in either square feet or square metres, so the estimate works with metric or imperial measurements. The square count is always computed from the area in square feet, since a “square” is defined as 100 ft².

Worked example

Take a wall 40 ft long and 10 ft high (about 12.19 m by 3.05 m) with 40 ft² of openings (roughly 3.72 m² of doors and windows) and a 10% waste allowance.

• Gross area: $40 \times 10 = 400 \text{ ft}^2$ (about 37.16 m²).
• Net area: $(400 - 40) \times 1.1 = 396 \text{ ft}^2$ (about 36.79 m²).
• Squares: $\lceil 396 / 100 \rceil = 4$ squares.

So this wall needs 4 squares of siding. Since 1 square = 100 ft² = 9.2903 m², four squares cover 400 ft² (37.16 m²), leaving a small buffer over the 396 ft² net area — exactly the point of rounding up.

Without any openings, the same 40 ft × 10 ft wall with 10% waste needs $(400 - 0) \times 1.1 = 440 \text{ ft}^2$, which is $\lceil 440 / 100 \rceil = 5$ squares.

Practical notes

• Waste allowance matters. A 10% allowance suits simple gables and straightforward walls. Bump it to 15% or more for many corners, dormers, or diagonal cuts, where offcuts add up.
• Measure openings honestly. Subtracting large doors, garage openings, and picture windows can save a full square. Small windows are often left in as buffer, effectively adding to your waste margin.
• One square = 100 ft². This is the trade convention for siding, roofing, and similar cladding. It is a unit of area coverage, not a physical panel — a box of siding will state how many square feet (or squares) it covers.
• Pair it with related tools. Confirm your wall area with the square footage calculator, estimate interior board with the drywall calculator, or plan the finish coat with the paint calculator.

Frequently asked questions

How many square feet are in a square of siding? Exactly 100 square feet, which is about 9.2903 square metres. To convert net area to squares, divide the area in square feet by 100 and round up.

Should I subtract windows and doors? Subtract large openings — a garage door or a wide window is worth a full square. Leaving small windows in the estimate gives you extra material, which acts as a built-in waste buffer.

How much waste should I add? Around 10% for plain walls, and 15% or more for complex elevations with many corners, gables, or angled cuts that produce more offcuts.