How many squares of siding do I need?

Work out the wall area — perimeter times wall height, plus the gable triangles — subtract the windows and doors, add about 10% for waste, and divide by 100. A 140 ft perimeter at 9 ft is 1,260 ft²; with two 30 ft gables 7.5 ft high (225 ft²) and 96 ft² of openings, that is 1,389 ft², 1,528 with waste: 15.3 squares.

How do I work out the area of a gable?

A gable is a triangle: its width times its height from the top of the wall to the peak, divided by two. A 30 ft wide gable rising 7.5 ft is 30 × 7.5 ÷ 2 = 112.5 ft². In metric, a 9 m gable rising 2.25 m is 10.1 m².

Should I subtract windows and doors from siding?

Yes, but not every small one. Big openings — doors, patio doors, picture windows — are worth taking off. Many installers leave small windows in as a built-in allowance for the extra cutting around them. The calculator subtracts what you type, so leave the count at zero for a bare-wall figure.

How much J-channel and starter strip do I need?

Starter runs along the bottom of every wall, so it is roughly the perimeter. J-channel goes around the windows and doors and up the rakes of the gables. A 3 × 4 ft window takes 14 ft of J around it; the calculator adds the openings and the rakes for an estimate, before laps and waste.

Siding calculator

Wall length (perimeter)

All the walls you are siding, end to end.

Wall height

Bottom of the siding to the eave.

Gables

gables

Gable width

Gable height

Top of the wall to the peak.

Windows and doors

openings

Opening width (typical)

Opening height (typical)

Coverage per box

ft²/box

From the carton or the product sheet.

Waste factor

— boxes to buy

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Show the working — with your numbers

The count is wall area plus gables, less the openings you type, plus waste, divided by the coverage on the box. It does not count outside and inside corner posts, soffit, fascia or the extra waste of dormers and bump-outs, and the trim lengths are a rough estimate before laps.

Type the wall perimeter and height, the gables, a count of windows and doors with a typical size, and the coverage on the box. You get the boxes to buy, the area in squares, and a first estimate of starter strip and J-channel.

Worked example

Imperial. A 40 × 30 ft house has a 140 ft perimeter; at 9 ft that is 1,260 ft² of wall. Two 30 ft gables 7.5 ft high add 2 × 30 × 7.5 ÷ 2 = 225 ft². Eight 3 × 4 ft openings take off 96 ft². Net 1,389 ft², plus 10% = 1,527.9 ft² — 15.3 squares. At 200 ft² a box that is 7.64, so 8 boxes. Starter: 140 ft. J-channel: 8 × 14 = 112 ft around the openings plus about 67 ft up the four rakes, 179 ft.

Metric. A 12 × 9 m house has a 42 m perimeter; at 2.7 m that is 113.4 m². Two 9 m gables 2.25 m high add 20.25 m². Eight 0.9 × 1.2 m openings take off 8.64 m². Net 125.01 m², plus 10% = 137.5 m² (14.8 squares). At 18.6 m² a box that is 7.39, so 8 boxes.

On the job

Squares are how it is priced. A square is 100 ft² of wall (9.29 m²). Suppliers quote siding, and many installers quote labour, by the square — and a box of vinyl is often two squares, but check the carton for the product you are buying.

Waste depends on the house. A plain box of a house is fine at 10%. Lots of gables, dormers and short walls between windows push it toward 15%, because the offcuts are too short to use elsewhere.

Trim is its own order. Starter, J-channel, corner posts, window heads and soffit are sold by the piece. Count the outside and inside corners separately and divide each trim length by the stick length the supplier stocks.

What went wrong?

Quick referencefor siding

Wall area: perimeter × height + gables (width × height ÷ 2) − openings
Squares: area ÷ 100 ft² · 1 square = 9.29 m² · 1 m² = 10.76 ft²
Boxes: (net area × (1 + waste)) ÷ coverage per box, rounded up
J around an opening: 2 × (width + height) — a 3 × 4 ft window takes 14 ft (0.9 × 1.2 m: 4.2 m)
Gable rake: each side √((width ÷ 2)² + height²) — 30 ft × 7.5 ft: 16.8 ft a side

The formula

boxes = ((P × H + G × W_g × H_g ÷ 2 − N × w × h) × (1 + waste)) ÷ coverage

P and H are the wall perimeter and height, G the number of gables of width W_g and height H_g, and N openings of w × h. Squares are the same padded area over 100 ft². The trim estimate is the perimeter for starter and the opening perimeters plus the gable rakes for J-channel.

Questions

How many squares of siding do I need?: Work out the wall area — perimeter times wall height, plus the gable triangles — subtract the windows and doors, add about 10% for waste, and divide by 100. A 140 ft perimeter at 9 ft is 1,260 ft²; with two 30 ft gables 7.5 ft high (225 ft²) and 96 ft² of openings, that is 1,389 ft², 1,528 with waste: 15.3 squares.
How do I work out the area of a gable?: A gable is a triangle: its width times its height from the top of the wall to the peak, divided by two. A 30 ft wide gable rising 7.5 ft is 30 × 7.5 ÷ 2 = 112.5 ft². In metric, a 9 m gable rising 2.25 m is 10.1 m².
Should I subtract windows and doors from siding?: Yes, but not every small one. Big openings — doors, patio doors, picture windows — are worth taking off. Many installers leave small windows in as a built-in allowance for the extra cutting around them. The calculator subtracts what you type, so leave the count at zero for a bare-wall figure.
How much J-channel and starter strip do I need?: Starter runs along the bottom of every wall, so it is roughly the perimeter. J-channel goes around the windows and doors and up the rakes of the gables. A 3 × 4 ft window takes 14 ft of J around it; the calculator adds the openings and the rakes for an estimate, before laps and waste.

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