How to measure a house for siding
Measure every wall to be sided and every gable, then subtract the openings; the calculator wants four numbers from you:
- Perimeter: the total length of all walls, measured at the foundation.
- Wall height: from the bottom of the siding to the soffit.
- Gables: the triangle above the wall is half the gable width times its rise; the rise is half the width × pitch ÷ 12. Find the pitch with the roof pitch calculator.
- Openings: count windows and doors; set a typical window size under More options.
Vinyl siding and lap siding: squares and boards
A vinyl siding calculator works in squares: a square of siding covers 100 sq ft of wall, and vinyl is sold by the square in cartons. A lap siding calculator also needs the exposure. Lap siding is sold by the piece, so divide the area by the exposure (the face each board shows) to get linear feet, then by the board length. Exposure, not board width, sets the count: a board 8-1/4 in wide laid at a 7 in exposure covers 7 in per course.
| Exposure | Linear feet | 12 ft boards |
|---|---|---|
| 4 in | 300 | 25 |
| 5 in | 240 | 20 |
| 6 in | 200 | 17 |
| 7 in | 171 | 15 |
| 8 in | 150 | 13 |
Trim and accessories
- Starter strip runs along the bottom of every wall: one perimeter.
- Outside corner posts: one wall height per corner.
- J-channel wraps windows and doors: the perimeter of each window, plus the top and sides of each door.
- Soffit and fascia are separate; measure the eaves and rakes as for the roof calculator.
Before you order
Siding goes over a water-resistive barrier and is fastened per the manufacturer and the code's wall-covering chapter[1]. The siding maker's carton states the exposure and coverage for each product; use those numbers in the calculator. For vertical boards with battens, use the board and batten calculator, and for the sheathing behind the siding, the plywood calculator.
Common siding estimating mistakes
Siding orders come up short for a few reasons:
- Board width instead of exposure. Each course covers only its exposure.
- Forgetting the gables. Two 28 ft gables at 6/12 add 196 sq ft.
- No trim list. Starter strip, corner posts and J-channel are ordered by length, not by the square.
- No waste allowance. Cuts around windows and at gable angles waste material; 10% is a common starting point.
Frequently asked questions
How does a lap siding calculator use the exposure?
Each course covers its exposure, not its full width, so linear feet = area × 12 ÷ exposure in inches. At a 7 in exposure, 100 sq ft takes about 171 linear feet.
Does this work as a vinyl siding calculator?
Yes. Vinyl siding is sold by the square (100 sq ft), so the squares figure is your order; add starter strip, corner posts and J-channel from the list.
How do I calculate how much siding I need?
Wall length × height, plus gable triangles, minus windows and doors, then add about 10% waste. Divide by 100 for squares.
How many square feet are in a square of siding?
100 square feet.
How do I calculate the area of a gable?
Half the gable width times its height. A 28 ft gable at 6/12 rises 7 ft, so 28 × 7 ÷ 2 = 98 sq ft.
How much lap siding for 1,000 sq ft?
At a 7 in exposure, about 1,714 linear feet before waste, or 143 boards 12 ft long.