How siding is calculated
Siding is ordered by area. In the US and Canada it is counted in squares, and one square is 100 sq ft of wall. That unit comes from roofing and lets you compare boxes of vinyl, bundles of cedar and planks of fiber cement on the same basis.
- Walls. Add the lengths of all the walls you will side and multiply by the wall height up to the eaves.
- Gables. Each gable end is a triangle: width × height ÷ 2.
- Openings. Subtract doors, windows and garage doors.
- Waste. Add a percentage, then divide by 100 to get squares.
squares = ((wall length × height) + gables − openings) × (1 + waste %) ÷ 100
Worked example
A 40 × 30 ft house has 140 ft of wall, 8 ft high: 1,120 sq ft. Two gable ends, each 30 ft wide and 7.5 ft tall, add 225 sq ft. Doors and windows total 125 sq ft, which leaves 1,220 sq ft. With 10 percent extra you need 1,342 sq ft, or 13.42 squares, so order 14 squares. In metric that is 124.7 m² including waste.
Gable heights from roof pitch
If you cannot reach the peak, work out the gable height from the roof pitch: half the gable width × pitch ÷ 12. A 30 ft wide gable with a 6/12 roof is 15 × 6 ÷ 12 = 7.5 ft tall.
How much waste to add
| House | Extra to add |
|---|---|
| Simple rectangle, few openings | 10% |
| Several gables, many windows | 15% |
| Dormers, bump-outs, angled walls | 15 to 20% |
These are rules of thumb. Siding cut for gables and around windows leaves offcuts that rarely fit elsewhere, so complex walls waste more. Check the maker's coverage per box or piece, because exposure and profile vary.
Converting squares to pieces
Lap siding covers its exposure, not its full width. A plank covers its length × exposure. For example, a 12 ft fiber cement plank installed with 7 in exposed covers 7 sq ft, so a square takes about 15 planks (14.29 before rounding up). Vinyl siding is usually sold in cartons that state their coverage in squares on the label.
How common siding types are sold
| Siding | Sold by | What to check |
|---|---|---|
| Vinyl | Cartons | Coverage per carton printed on the label |
| Fiber cement planks | Pieces | Plank length and exposure |
| Wood clapboard or bevel | Linear feet or pieces | Board width, overlap and exposure |
| Engineered wood lap | Pieces | Plank length and exposure |
| Cedar shakes and shingles | Bundles | Coverage per bundle at your chosen exposure |
Whatever the material, work out the area first with this calculator, then use the coverage figure for your product to turn squares into cartons, pieces or bundles.
Measuring the walls
Measure each wall from the bottom edge of the siding, usually just above the foundation, up to the soffit. Walk around the house and write down each length, including garage walls, porches and bump-outs. Areas under a porch roof or behind a deck often get siding too, so do not forget them.
What else to order
- Outside and inside corner posts: one per corner, as long as the wall height.
- J-channel around windows and doors and along the eaves and gable rakes.
- Starter strip along the bottom of every wall: equal to the total wall length.
- House wrap or weather-resistive barrier behind the siding, as your code requires.
- Soffit and fascia, measured separately along the eaves.
Working on gables means ladders or scaffolding. Set them on firm, level ground and keep three points of contact. Removing old siding can expose lead paint on older homes; test before you sand or scrape.
Frequently asked questions
How many square feet are in a square of siding?
One square is 100 sq ft, about 9.29 m². A wall area of 1,342 sq ft is 13.42 squares, so you would order 14.
Should I subtract windows and doors?
Yes, subtract doors, windows and garage doors, then add a waste allowance. Small windows save little material because the cuts around them create offcuts.
How do I calculate the area of a gable?
Multiply the gable width by its height and divide by 2. A 30 ft wide gable that is 7.5 ft tall is 112.5 sq ft.