Siding Calculator

Net wall area in, siding quantities out, trim counted on its own

Enter each wall with its gable if it has one, deduct the openings honestly, and choose how the siding is sold. This calculator returns the net area, the material in squares (the siding trade’s 100-square-foot unit) with waste applied and the order rounded up the way yards sell it, or, for lap boards, the linear feet and board count at your exposure. Trim, corners and starter are counted separately at the bottom, because they are bought by the piece and the foot, not by the square, and folding them into an area is how estimates quietly go wrong.

Walls (feet)
Small windows under about 10 sq ft are often left in the area by installers, as the cut waste replaces them. Deduct what you truly will not cover.
Trim and accessories (counted separately)

Disclaimer: This calculator gives estimates for general information only. It is not financial, medical or other professional advice. Quantities are planning figures; your supplier and installer confirm the final order against the product’s actual coverage.

What this calculator does

Siding is bought three ways at once: covering by area, boards by length, and trim by the piece. Mixing the three is the standard estimating error, so this tool keeps them apart on purpose. Walls are measured as rectangles plus their gable triangles, openings are deducted as a single honest total, and the net facade area converts two ways: into squares (one square covers 100 sq ft, the unit vinyl and fibre cement are priced and packed in) or into lap-board linear feet at the exposure you set, because a board shows only its exposed face and the hidden overlap is coverage you pay for but never see. Waste is then applied openly, purchase quantities are rounded up to sellable units, and trim is totalled from your own counts, not guessed from the area.

How to use it

  • Measure walls at their full rectangles. Width times height from foundation to eaves, gable added as half width times its height. Corners overlap slightly in this method; the waste allowance absorbs it, and measuring shy is the expensive direction.
  • Deduct openings the way your installer does. Doors and large windows come out; small windows are a judgement call many installers leave in. The hint under the field states the convention; matching your installer’s convention beats theoretical purity.
  • Set exposure from the product, not the habit. A 12-inch plank might expose 10 inches or 8 depending on the specified overlap; the linear feet swing by a quarter between those. The manufacturer’s stated exposure for the exact profile is the number that belongs in the field.
  • Count trim by walking the house. Corner posts equal the house’s corners at eaves height; starter runs the base perimeter; J-channel frames every opening. Five minutes with a tape produces these, and no area formula can.

The method

  • Gross area = sum over walls of (width × height) + (width × gable height / 2) where a gable is entered. Net area = gross − openings.
  • Squares = net area / 100, then × (1 + waste%). Order quantity rounds up to the next whole square (and product is often packed as 2 squares per carton, which suppliers will apply).
  • Lap boards: linear feet = net area (in sq inches) / exposure (inches), then × (1 + waste%). Boards = linear feet / board length, rounded up.
  • Trim is summed from counts and lengths exactly as entered; corner posts are priced by the piece at standard 10 ft lengths.

Worked examples

The gabled wall. One wall 40 ft wide and 10 ft tall with an 8 ft gable: 400 + (40 × 8 / 2) = 560 sq ft gross. Deduct 100 sq ft of openings: 460 net. At 10 percent waste the job needs 506 sq ft of coverage, which is 5.06 squares, ordered as 6 squares when sold by the whole square. The gap between 5.06 and 6 is not error; it is how the material is packaged, and seeing it on the estimate is the point of printing both.

The same wall in lap boards. At an 8-inch exposure, the 460 sq ft net needs 460 × 144 / 8 = 8,280 raw linear inches of course, or 690 linear feet before waste, 759 with 10 percent, which is 64 boards of 12 ft (63.25 rounded up). A 10-inch exposure on the same wall drops the order by roughly a sixth, which is why the exposure field earns its keep.

Reading your results

Read net area as the truth the quantities stand on; if it surprises you, the walls and gables list is where to re-measure. Read exact squares against the order quantity as a packaging lesson rather than a discrepancy. And read the trim lines as deliberately separate money: on real jobs trim, corners and starter are a substantial share of the materials bill, and an estimate that hides them inside an area rate is one that will be re-explained at the till.

Limitations

  • Product coverage varies by profile. Vinyl panels, fibre-cement planks and wood boards all state their own coverage and exposure; the calculator’s squares are exact geometry, but the product’s stated coverage per carton is the final word and can differ slightly.
  • Gables are triangles here. A gambrel, a clipped gable or dormers breaking the slope need the shape split; enter the triangle that fits best and let waste carry the difference, or measure the extra faces as their own walls.
  • Labour is not a material. This page quantifies what to buy. Installation time depends on storeys, access, tear-off and detailing in ways no area converts to, and any labour figure belongs to a quote, not a calculator.
  • Openings convention differs by installer. Whether small windows are deducted changes net area by a few percent either way; the field is yours to set by the convention of whoever is installing.

Frequently asked questions

What is a square of siding?

One square is 100 square feet of coverage, the unit the siding trade prices, packs and quotes in, borrowed from roofing. A house needing 1,850 sq ft of coverage needs 18.5 squares before waste and packaging round it. Suppliers quote per square even for products sold by the carton, so converting your net area into squares is the step that makes prices comparable between products and yards.

How much waste should I allow?

Ten percent is the working standard for straightforward walls; 12 to 15 for gable-heavy, cut-up elevations or diagonal and specialty patterns that generate offcuts. Waste covers cutting, breakage, the occasional mis-measured course and the lengths left over at each wall’s end. Skimping to 5 percent on a complex elevation reliably produces the most expensive outcome: a second delivery and a dye-lot that almost matches.

Vinyl, wood or fibre cement: does the calculator care?

Only through how each is sold and exposed. Vinyl typically comes in panels priced by the square with its own trim ecosystem (the trim fields are modelled on it). Wood and fibre-cement lap boards sell by length at stated exposures, which is the lap mode. The geometry and waste logic are identical; the product’s data sheet supplies the exposure and coverage figures that make the output product-specific.

Do I deduct the garage and covered porches?

Deduct any face you are not siding, as part of the openings total if it interrupts a wall, or by simply not entering that wall. Covered porches that do get sided are measured as their own short walls. The rule is the surveyor’s rule: every face entered once, at its true size, and nothing counted twice, which the wall-by-wall list makes easy to check before the order is placed.

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Method: wall rectangles plus gable triangles, less openings, converted to squares at 100 sq ft or to lap-board linear feet at the entered exposure, with waste applied openly and purchase quantities rounded up; trim counted independently. Last reviewed: 15 October 2026.