Siding Calculator

How much siding do I need? Start with the walls, not the floor plan

Siding is sold by wall area, not by house size. A 2,000 sq ft ranch and a 2,000 sq ft two-storey need very different orders, because the two-storey has half the footprint and twice the height. So the only measurement that matters is the perimeter of the exterior walls multiplied by the height you are covering, plus the triangles of any gable ends, minus the openings you will not be siding over.

Walk the house with a tape and add up each wall run: front, side, back, side. Bump-outs and bays add their own runs. That total is the perimeter. For height, measure from the bottom of the starter strip to the soffit — the eave height — not to the roof peak. Gables are handled separately because they are triangles.

Worked example: a 140 ft colonial

Take a two-storey colonial with a 140 ft perimeter, 9 ft eave walls, two gable ends 30 ft wide and 8 ft to the peak, two exterior doors, eight windows and four outside corners.

Field walls: 140 x 9 = 1,260 sq ft. Gables: each is 30 x 8 / 2 = 120 sq ft, so two add 240. Gross wall area is 1,500 sq ft. Deduct the openings at 21 sq ft per door and 15 sq ft per window: 42 + 120 = 162 sq ft. Net area is 1,338 sq ft, or 13.4 squares bare. Add a 10 percent waste allowance and the order becomes 14.7 squares — about 30 boxes of vinyl at the usual two-boxes-per-square packaging.

The accessories follow from the same measurements. Starter strip runs the full perimeter: 140 linear feet. Corner posts: four corners at 9 ft of wall, and since posts come in 10 ft lengths, that is four posts. J-channel is the long one: roughly 17 ft around each door and 14 ft around each window (2 x 17 = 34, plus 8 x 14 = 112, so 146 lf), plus both rakes of each gable. A 30 ft wide, 8 ft tall gable has rakes of sqrt(15² + 8²) = 17 ft each, so two gables add 68 lf. Total J-channel about 214 linear feet, which is eighteen 12 ft 6 in sticks.

Why the deduction rule is deliberately stingy

A standard 3 ft x 6 ft 8 in door is 20 sq ft and a common double-hung window is about 15. Many estimators deduct nothing at all for openings, because every opening creates cut waste at its edges that roughly cancels the area saved. This calculator takes the middle road: it deducts a modest 21 and 15 sq ft, then adds waste on top. If your house has a wall of glass — a big picture window or sliders — measure those openings and treat the extra as part of your waste cushion rather than deducting them fully.

What drives the waste percentage

Ten percent covers a plain rectangular house with long, uninterrupted runs. Fifteen percent is the realistic number when you have several gables, dormers, bay windows, staggered rooflines or a lot of short walls, because each cut leaves an offcut too short to reuse. Two other factors push it up: a strong colour or woodgrain pattern that has to be matched across a wall, and a first-time DIY install, where the first two walls tend to teach you an expensive lesson.

Order the whole job at once. Vinyl colour varies subtly between production lots, and a mid-project top-up in a slightly different batch shows up in daylight on a large wall.

Limitations worth knowing before you order

This is a materials estimate, not a quote. It does not price labour, tear-off, disposal of old siding, insulation board, house wrap, soffit and fascia, or the flashing that belongs at every window head and wall-roof junction. It assumes a single height for all walls, so if one side is a walkout basement with 18 ft of exposure, run that wall as its own calculation and add the results. It also assumes rectangular walls plus simple triangular gables — turrets, curved walls and hip-and-valley cut-ups need takeoff by hand.

Finally, take the square figure to the supplier rather than the box figure. Box coverage differs by manufacturer and profile; the square count is the number that translates cleanly across every brand on the shelf.