Siding Calculator
Estimate siding for walls and gables, less windows and doors. Vinyl in squares and boxes or lap planks by exposure, plus J-channel, starter strip and corner posts.
How to Use
- Choose vinyl (sold in squares of 100 ft²) or lap planks (fiber cement or wood, counted by exposure).
- Enter the wall perimeter and height, then the number of gables with their width and rise.
- Enter how many windows, doors and garage doors there are; standard sizes are subtracted.
- Enter the outside corners and a waste allowance: 10% for simple walls, 15% with many openings or gables.
- Read the siding to buy, plus the J-channel or trim, starter strip and corner posts.
Worked Example
A 30 × 40 ft house with 9 ft walls, two 30 ft gables at 6/12 (7½ ft rise), 12 windows and 2 doors. Walls: 140 ft × 9 ft = 1,260 ft². Gables: 2 × ½ × 30 × 7.5 = 225 ft². Gross 1,485 ft². Openings: 12 windows × 15 ft² + 2 doors × 21 ft² = 222 ft². Net 1,263 ft²; with 10% waste, 1,389 ft².
Vinyl: 1,389 ÷ 100 = 13.9, so 14 squares, 7 boxes of 2 squares. Fiber cement at 7 in exposure on 12 ft planks: each covers 7 ft², so 1,389 ÷ 7 = 198.5, 199 planks. J-channel: 12 windows × 16 ft + 2 doors × 17 ft + 4 rakes × 16.8 ft = 293 ft, 26 lengths of 12½ ft with 10% extra. Starter: 140 ft less 6 ft of doorways = 134 ft, 11 lengths. Corner posts: 4.
The common mistake: forgetting the gables. Two gables on this house are 225 ft², 2¼ squares, about 15% of the job. Measuring only perimeter × wall height leaves you short on the last day.
Show Work
Formulas
Siding Then and Now
Clapboards, overlapping boards split or sawn thin at one edge, have covered timber houses in North America and northern Europe for centuries; the exposure, the band left showing, is still the number every lap siding is laid by. Aluminium siding arrived after the Second World War, and vinyl, introduced in the late 1950s, became the most common cladding on new American houses by the 1990s.
Fiber-cement planks, cellulose fibre in cement, were popularised by James Hardie in the 1980s and look like painted wood but do not rot or burn. They are heavy and are cut with dust-controlled tools, and like vinyl they are sold by the piece rather than the square.
About This Calculator
This calculator estimates siding for the walls and gables of a building: the net area after standard window and door sizes, vinyl in squares and boxes or lap planks by exposure and length, J-channel or trim around openings and rakes, starter strip and corner posts, in feet or metres. It draws a gable end with the courses to scale.
Everything runs in your browser; nothing is sent anywhere. Openings are taken at standard sizes; for unusual windows, adjust the waste. Soffit, fascia and house wrap are separate items.
Related calculators: Paint Calculator, Roofing Calculator, and Trim & Baseboard Calculator.
Frequently Asked Questions
What is a square of siding?
A square covers 100 ft² of wall (9.29 m²). Vinyl siding is sold in boxes of about two squares. A 1,263 ft² house plus 10% waste needs 13.9, so 14 squares, which is 7 boxes.
How do I measure a house for siding?
Measure around the outside to get the perimeter and multiply by the wall height from the foundation to the soffit. Add each gable as a triangle, half the width times the rise. Subtract windows and doors, then add waste. Dormers, bays and chimneys are measured separately and added.
How many lap siding planks do I need?
Each plank covers its length times its exposure, the part left showing. A 12 ft plank 8¼ in wide laid at 7 in exposure covers 7 ft². For 1,389 ft² with waste, that is 199 planks. Lay the first course on a starter strip so it kicks out at the same angle as the rest.
How much J-channel do I need?
J-channel runs around every window (top and both sides, plus the bottom unless you use under-sill trim), along both sides and the top of each door, and up each rake where the siding meets the roof. With 12 windows, 2 doors and two 30 ft gables, that is about 293 ft, 26 lengths of 12½ ft with 10% extra.
Should I subtract windows and doors?
Yes for large ones, but many installers subtract only openings over about 10 ft² and keep the rest as extra waste, because the pieces cut out around small windows are not reusable. If you subtract everything, use at least 10% waste.
How do I use the Siding Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
Whole house, vinyl
30×40 ft, 9 ft walls, two 30 ft gables at 6/12, 12 windows, 2 doors: 14 squares, 7 boxes.
Fiber cement
The same house in 12 ft planks at 7 in exposure: 199 planks.
Garage
24×24 ft, 8 ft walls, two gables, a garage door, a door and 2 windows: 749 ft² net, 9 squares of vinyl.
Gable end only
One 28 ft gable with 7 ft rise, no walls: 98 ft² net, 2 squares with waste.
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