Siding Calculator (squares, boxes & pieces incl. gables)

Understanding the siding calculator — concept and principle

This calculator estimates how much siding a house needs, including triangular gables and a waste allowance, as boxes, squares and pieces. Homeowners and contractors use it to price vinyl, fibre-cement or wood lap siding.

Walls plus gables: with the default inputs the result is 17.56 squares (9 boxes · 211 pieces).

How it is calculated

Gable area = number of gables × width × height ÷ 2.
Total area = net wall area + gable area; with waste = total × (1 + waste %).
Boxes = area with waste ÷ coverage per box, rounded up.
Squares = area with waste ÷ 100 ft².
Pieces = area with waste ÷ (exposure in ft × piece length), rounded up.
m² = ft² × 0.0929.

The principle

Siding is sold by the square, the area of 100 ft² of wall it covers. Walls are rectangles, but the gable ends of a pitched roof are triangles, half of width times height. Each piece covers only its exposure, the part left visible below the course above, times its length. A waste allowance covers cuts at corners, windows, doors and gable slopes, which are larger on houses with many openings.

Worked example

Default inputs: net walls 1,400 ft², 2 gables 28 ft wide and 7 ft high, 8 in exposure, 12.5 ft pieces, 200 ft² boxes, 10% waste.

Gables = 2 × 28 × 7 ÷ 2 = 196 ft².
Total = 1,400 + 196 = 1,596 ft² (148.3 m²); with waste = 1,755.6 ft².
Squares = 17.56.
Boxes = 1,755.6 ÷ 200 = 8.78, rounded up to 9.
Pieces = 1,755.6 ÷ (0.667 × 12.5) = 210.7, rounded up to 211.

Each point is the calculator re-run with only “Coverage per box (ft²)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Coverage per box (ft²)	Boxes to buy
50	36
100	18
150	12
200	9
250	8
300	6
400	5

How to read the result

Order trim separately: corner posts, J-channel, starter strip and soffit are bought by the running length. Raise the waste allowance for complex houses. Check that the coverage per box matches the product you are buying.

Common mistakes

Forgetting the gables.
Subtracting every small window, which leaves no margin for cut waste.
Using full piece width instead of exposure.
Ignoring trim and accessories in the budget.

Keep in mind: Installation must follow the manufacturer's instructions and local building codes, including fire, moisture and fastening requirements. This is an estimate, not a quote.

Frequently asked questions

How many squares of siding for 1,596 ft²?

About 17.6 squares including 10% waste.

How do I calculate gable area?

Width × height ÷ 2 for each triangular gable.

What is a square of siding?

The amount that covers 100 ft² of wall.

Sources: NIST Special Publication 811 (2008), Appendix B (1 ft² = 0.09290304 m²)

Checked 9 Nov 2026. Area of a rectangle and triangle; the square (100 ft²) is the trade unit for roofing and siding; 1 ft² = 0.09290304 m² (NIST SP 811).

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