Siding Calculator
Enter your house size, wall height and roof pitch to get the siding area in squares and the number of cartons to buy. You also get the trim list: starter strip, J-channel and corner posts, with waste included and an estimated material cost.
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Siding area = perimeter × wall height + gable triangles − openings. Add 10% waste and divide by 100 for squares. A 40 × 30 ft house with 8 ft walls, two 6/12 gables and 12 openings of 15 sq ft has 1,165 sq ft of wall. With waste that is 12.8 squares, so buy 7 cartons of 2 squares, plus 13 starter strips, 30 J-channels and 4 outside corner posts.
How to use this siding calculator
- Measure the house outside. Length and width of the walls, and the wall height from the bottom of the siding to the soffit.
- Pick the roof pitch and number of gables. A gable roof has a triangle of wall at each end. A hip roof has none, so choose No gables.
- Count windows and doors and enter their average size. A 3 × 5 ft window is 15 sq ft and a standard entry door about 21 sq ft.
- Count corners. A plain rectangle has 4 outside corners. Each L or bump-out adds outside and inside corners.
- Already measured? Switch to Measured wall area and enter the net area and trim lengths directly.
- Read your order: squares, cartons, cost, and every trim piece to buy with the siding.
How to calculate siding squares
Siding is sold by the square, which is 100 sq ft of wall. Work out the rectangular walls, add the gables, then take out the openings.
The gable rise is half the house width × pitch ÷ 12. On a 30 ft wide house with a 6/12 roof, the rise is 15 × 6 ÷ 12 = 7.5 ft and each gable triangle is 30 × 7.5 ÷ 2 = 112.5 sq ft.
| House width | 4/12 gable | 6/12 gable | 8/12 gable | 12/12 gable |
|---|---|---|---|---|
| 24 ft | 48 sq ft | 72 sq ft | 96 sq ft | 144 sq ft |
| 28 ft | 65 sq ft | 98 sq ft | 131 sq ft | 196 sq ft |
| 30 ft | 75 sq ft | 113 sq ft | 150 sq ft | 225 sq ft |
| 32 ft | 85 sq ft | 128 sq ft | 171 sq ft | 256 sq ft |
| 36 ft | 108 sq ft | 162 sq ft | 216 sq ft | 324 sq ft |
Area of one gable triangle. Multiply by the number of gable ends you are siding.
Siding coverage by type
| Siding | How it is sold | Typical coverage |
|---|---|---|
| Vinyl lap siding | Cartons | Usually 2 squares (200 sq ft) per carton |
| Fiber cement lap (8¼ in plank, 7 in exposure) | 12 ft planks | 7 sq ft per plank, about 14.3 planks per square |
| Engineered wood lap | 16 ft planks | Depends on width and exposure, check the label |
| Steel and aluminum lap | Cartons | Varies by profile, check the carton |
Coverage varies by profile and brand, so check the carton or product sheet before you order.
Trim and accessories you need with siding
- Starter stripRuns along the bottom of every wall to lock in the first course. The calculator uses the house perimeter plus 5% and 12 ft pieces.
- J-channelSurrounds windows and doors, runs under the soffit and up the gable rakes. Typically sold in 12 ft 6 in pieces. The count adds 10% for laps and miter cuts.
- Outside corner postsOne per outside corner per 10 ft of height. Posts are typically 10 ft long, so a two-story corner needs two.
- Inside corner postsWhere two walls meet in an inside corner, such as an L-shaped house or a bump-out.
- Also plan forHouse wrap, undersill (utility) trim under windows, soffit and fascia, and corrosion-resistant nails. Follow the siding manufacturer’s fastening instructions.
Worked example
- House 40 × 30 ft, 8 ft walls
- 2 × 70 × 8 = 1,120 sq ft
- Two 6/12 gables (7.5 ft rise)
- 2 × 30 × 7.5 ÷ 2 = 225 sq ft
- 12 openings of 15 sq ft
- 1,345 − 180 = 1,165 sq ft
- Add 10% waste
- 1,282 sq ft = 12.8 squares
- Cartons of 2 squares
- 7 cartons (14 squares) ≈ $1,820 at $130 per square
- Starter strip (140 ft + 5%)
- 13 pieces of 12 ft
- J-channel (333 ft + 10%)
- 30 pieces of 12 ft 6 in
- Corner posts (4 corners, 8 ft walls)
- 4 outside posts
Frequently asked questions
How do I calculate how much siding I need?
Perimeter × wall height, plus the gable triangles (width × rise ÷ 2), minus windows and doors. Add about 10% waste and divide by 100 for squares.
What is a square of siding?
100 sq ft of wall coverage. Vinyl siding usually comes in cartons of 2 squares. Check the carton for the exact coverage.
How many squares of siding for a 40 x 30 house?
With 8 ft walls, two 6/12 gables and 12 openings of 15 sq ft, about 1,165 sq ft of wall. With 10% waste that is 12.8 squares, or 7 cartons.
Should I subtract windows and doors from siding?
Yes for doors, garage doors and large windows. Many installers leave small windows in as extra waste, since the cuts around them make offcuts anyway.
How much waste should I add for siding?
About 10% for a simple rectangular house, 12–15% for many corners, gables, dormers or short walls.
How much J-channel do I need?
Measure around windows and doors, under the soffit and up each rake. Add about 10% and divide by the piece length, typically 12 ft 6 in.
What this calculator assumes
- A rectangular house with gables on the width walls. For other shapes, add up each wall and use Measured wall area.
- J-channel runs around each opening (estimated from its area as a square), along the top of non-gable walls and up both rakes of each gable.
- Starter strip in 12 ft pieces, J-channel in 12 ft 6 in pieces and corner posts in 10 ft lengths. These are typical; check your product line.
- Prices are material estimates for the siding only, before tax and delivery, and exclude trim, labor and removal of old siding.