Siding calculator: squares, boxes, J-channel and corners

Siding, like roofing, is sold and priced by the square: 100 sq ft of wall covered. Vinyl comes in cartons of about two squares; fiber cement and engineered wood come as planks, and the coverage of a plank is its length times its exposure — a 12-ft fiber cement plank at a 7-inch exposure covers 7 sq ft.

Net area = (perimeter × wall height) + (gables × width × rise ÷ 2) − openings. Each gable is a triangle, so its area is half the base times the height from the eave line to the peak. Add waste to the net area, divide by 100 for squares, and divide by the product’s coverage for boxes or planks.

Starter strip runs the full perimeter at the bottom course. J-channel wraps windows, frames the sides and head of doors, and runs up both rakes of every gable, where the length is the sloped side — the square root of (half the width² + the rise²). Outside corner posts come in 10-ft lengths, one per corner per 10 ft of wall height. Soffit, fascia, inside corners and utility trim are separate lines.

Before siding goes on, the wall behind it has to be framed and sheathed — the stud calculator covers both — and on retrofit jobs an insulated sheathing or wrap may go under it; see the insulation calculator. To turn squares into a price with tear-off, wrap and trim labor, use the estimate builder or the siding estimating software pack.