How much siding do I need? Measuring walls, gables and openings

By Francesco Zinghinì

Siding is sold by the square too

Like roofing, siding uses the square — 100 sq ft of wall. The job is to find your net wall area, convert to squares, add waste, and then translate squares into whatever unit your product ships in (vinyl panels and cartons, for instance). Careful measuring here saves both under-ordering (a mid-job stop and a possible color-lot mismatch) and expensive over-ordering. The same square count also drives house wrap, trim and fasteners.

Step 1 — the rectangular walls

Measure the perimeter of the house and the wall height from foundation to eave. Multiply them for the gross rectangular wall area:

wall area = perimeter × height

Worked example: a 140 ft perimeter and 9 ft walls give 140 × 9 = 1,260 sq ft of rectangular wall. Measure story by story if wall heights differ, and include only walls that will be sided (skip a shared wall with an attached garage, say). On a two-story house, measure each story's height and add them, or use the total wall height in one pass if the perimeter is the same.

Step 2 — add the gables

Gable ends are the triangles under the roof at each end of a ridge. A triangle's area is half base times height: half the wall width times the gable's vertical height (from the eave line up to the ridge). A gable 30 ft wide and 5 ft tall is 0.5 × 30 × 5 = 75 sq ft; two of them add 150 sq ft. Add every gable, dormer cheek and other triangular area to the rectangular total. In our example, 1,260 + 150 = 1,410 sq ft gross. Don't skip the gables — on a steep-roofed house they can be a meaningful share of the wall.

Step 3 — subtract the openings

Subtract windows and doors so you do not pay to side over them:

net area = walls + gables − openings

Add up the square footage of each window and door (width × height in feet). Say they total 180 sq ft: 1,410 − 180 = 1,230 sq ft net. A common shortcut is to subtract only the large openings and let small ones get absorbed into the waste factor, but subtracting them all and using a proper waste percentage is cleaner and less likely to leave you short or long.

Step 4 — squares and waste

Convert to squares and add waste:

squares = net ÷ 100 × (1 + waste)

1,230 sq ft is 12.3 squares; at 10% waste that is 12.3 × 1.10 = 13.5 squares to order. Use 10% for a simple house and 15% for one that's cut up with many corners, dormers, angles and short runs, where offcuts pile up. The siding squares calculator runs all four steps at once.

From squares to panels, cartons and rolls

Squares then convert to the product unit. For vinyl, panel coverage is exposure/12 × panel length: a double-4 (8-inch exposure) 12 ft panel covers 8 sq ft, so 1,230 sq ft plus 10% waste is about 170 panels, roughly 8 cartons of 24 — see the vinyl panel calculator and the exposure chart. Panels per carton vary by product, so check the label. And don't forget house wrap, sized on gross wall area (openings are not deducted, since the wrap is installed over them and cut out afterward): 1,410 ÷ 900 = 2 rolls.

Different siding, different units

The square method covers area-based products like vinyl, fiber-cement lap and panel siding, but not everything is sold that way, so know your product's unit before you convert. Board-and-batten and some plank sidings are often figured by the linear foot per course, which means you need the wall's coverage width and the board's exposure, not just square footage. Shakes and shingles come by the square but at a specific exposure that changes coverage. Fiber-cement lap installs much like vinyl and maps cleanly to squares, but its planks are heavier and cut differently, affecting waste. The safe workflow is the same either way: nail down the net wall area in square feet first (walls + gables − openings), then translate into whichever unit your chosen product ships in — squares, planks, cartons or linear feet — using that product's coverage. Area is the common currency; the final unit conversion is product-specific.

Trim, accessories, and ordering

The square count covers the field, but a siding job also needs accessories measured by the linear foot: J-channel around windows and doors, corner posts at each outside and inside corner, starter strip along the bottom, and finish/undersill trim at the top. Tally those from your window and door counts and corner count. As with shingles, buy field siding from a single production lot so colors match, and keep a spare carton for future repairs — discontinued colors are hard to match later. All figures are planning estimates: measure carefully and get written quotes before ordering.