Siding Calculator

The formula

squares = ceil( (perimeter × height − doors×20 − windows×15) × (1+wastePct/100) / 100 )

perimeter

2 × (length + width) — this treats the two length/width fields as the FULL BUILDING'S footprint and sides ALL FOUR walls at once, not one wall in isolation. A single-wall job still needs both a length and a width entered, even if the width is only used to compute the returns at each end.

doors×20, windows×15

The same flat per-opening deductions in square feet that the wall-paint and drywall calculators use — not a measured opening, and not adjusted for siding-specific trim like J-channel, which sits on top of these deductions rather than inside them.

squares

One square equals 100 sq ft of net wall coverage — the SAME unit name roofing uses, but computed from vertical wall area here rather than sloped roof area, so a 'square' of siding and a 'square' of shingles are unrelated quantities that happen to share a name.

Where these numbers come from

20 sq ft per door, 15 sq ft per window

Trade convention shared with this site's paint and drywall calculators for a standard opening — an oversized garage door or a large picture window is undercounted by this flat figure the same way it would be for paint.

100 sq ft per square, coverage varying by material

Trade convention for the unit itself; how much actual product covers one square differs by material (vinyl panel overlap versus fiber-cement reveal), which this formula does not adjust for — it reports net square footage divided by 100 regardless of which material is being priced.

perimeter of all four walls, not a single wall's length

Settling this required reading the function rather than guessing from the field labels: because length and width are both required, this calculator always models a rectangular building's full wall perimeter. A quote for a single wall in isolation needs its own length treated as one side of a notional rectangle, or the total divided down by hand afterward.

Worked examples

Common mistakes

• Applying one flat coverage-per-square figure across different siding materials, which under- or over-orders depending on the actual product's reveal/overlap.
• Forgetting trim components (corner posts, J-channel, starter strip, soffit/fascia) which are separate line items from the field siding panels.
• Not adding waste for cutting around windows, doors, and outside corners, typically 10% on a simple wall and more on a wall with lots of openings.
• Ignoring that a wider fiber-cement reveal (more board exposed per course) uses less material per square foot than a narrower reveal — the choice itself changes the quantity needed.
• Not checking house-wrap/weather barrier condition before residing over it, since a full siding replacement is the practical point to address any underlying moisture barrier issues that would otherwise stay hidden for years.
• Mixing lot numbers on a visible elevation — like tile and shingles, siding color can vary subtly between production runs, and a mismatch is more visible on a large, unbroken wall than on a smaller surface.

Shopping summary

Buy field siding in squares rounded up from net wall area at your material's actual coverage rate plus 10% cutting waste, plus trim components (corner posts, J-channel, starter strip) by linear footage of corners, openings, and the wall base.

Worked example, and note that the calculator sizes a whole building rather than one wall: a 20 by 30 ft building with 9 ft walls has a 100 ft perimeter, so 900 sq ft gross. Three windows at 15 sq ft and one door at 20 sq ft bring that to 900 - 45 - 20 = 835 sq ft net. At 10% cutting waste, 835 x 1.1 = 919 sq ft, which is 10 squares once rounded up. If you are siding a single elevation rather than a whole building, enter that wall length and set the width small enough that the perimeter matches the run you actually mean to cover.