Siding Calculator

How the siding count works

Siding is one of the easier take-offs once you have the right area, and most of the mistakes happen before the arithmetic starts.

Wall area

Each elevation is length times height, measured to the top of the wall rather than to the roof. Four fields cover a rectangular house; for an L-shaped house, run the extra legs through as separate walls or use the fields twice.

Gables are triangles

gable area = 0.5 × width × rise to the peak. A 30 foot wide gable rising 6 feet to the ridge is 0.5 × 30 × 6, which is 90 square feet. Measure the rise from the top of the wall to the peak, not from the ground. Two gables on that house add 180 square feet, which is most of two squares of siding and the single most commonly forgotten number on the whole job.

Exposure, and why it is not the board width

Exposure is the part of a course you can see after the next course laps over it. An 8 inch lap board might only show 6 3/4 inches. It is exposure, not the board width, that decides how much wall a piece covers:

coverage per piece = piece length in inches × exposure ÷ 144

A 12 foot piece is 144 inches, so at a 6 inch exposure it covers exactly 6 square feet. That gives a tidy shortcut for 12 foot stock: the coverage in square feet is the same number as the exposure in inches. Divide your adjusted area by that figure and round up.

Squares and boxes

A square is 100 square feet of finished wall, and siding is priced by the square everywhere. Vinyl usually ships in boxes covering 2 squares, so the box count is your area divided by 200, rounded up. Ordering by the square and ordering by the box are not the same thing: a house needing 12.8 squares takes 7 boxes, which is 14 squares of material, so there will be some left over.

Waste

10 percent covers a plain rectangular house with a handful of windows. Use 15 percent on anything cut up: lots of openings, dormers, several gables, or an elevation broken by a bump-out. Every one of those turns a full-length piece into two pieces with an offcut in between, and the offcuts get shorter as the wall gets busier.

Starter strip and J-channel

Starter strip runs along the bottom of every wall, once, so it is the total of your wall lengths. J-channel goes around every opening, and the figure here is the perimeter of the openings you entered. Add the rake on each side of every gable and the run under the eave, because those are real linear feet this calculator does not know about.

What goes under it

Siding goes over sheathing and house wrap, and both are figured from the same wall area. If the sheathing is going on at the same time, run the numbers through the plywood and sheathing calculator, and use the wall framing calculator if the walls are being built rather than re-clad.