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Roofing & Siding

Siding Calculator

Squares of siding, trim and starter strip for a house, with openings deducted.

Your measurements

Work in whole wall faces; gables are entered separately.

Walls
Half the gable width times its height, per gable. Leave at zero for a flat roof.
Openings and material
Ten percent for a simple rectangular house; fifteen where there are dormers, bays or many corners.

Your result

Sample values

14.6

Squares

1,326

Net area, ft²

150

Feet of starter strip

280

Feet of J-channel

Includes gables and a 10% waste allowance. One square is 100 ft².

What to order

Item Quantity
Siding 14.6 squares
Starter strip 150 ft
J-channel, openings 280 ft
Outside corner posts 4
Utility trim 150 ft

How to use this calculator

Measure the perimeter of the house at ground level and the wall height to the eaves, then add the gable areas separately. A gable is close enough to half its width times its height for estimating, and that is the figure to enter.

Deduct doors and windows at standard sizes. Siding is sold by the square, which is one hundred square feet, so the headline figure converts straight into what you will be quoted.

Formula

Wall area = Perimeter × Height

Net area = Wall area + Gables − Openings

Squares = Net area ÷ 100 × (1 + Waste)

Square
100 ft² of finished coverage
Gable
about half the width times the height

Trim lengths are estimated from the perimeter and the opening count.

Worked example

A house with a 150 ft perimeter, 9 ft walls, 120 ft² of gable, two doors and twelve windows, 10% waste.

  1. 150 ft × 9 ft = 1,350 ft² of wall
  2. 1,350 + 120 gable = 1,470 ft²
  3. Openings: (2 × 21) + (12 × 12) = 186 ft²
  4. 1,470 − 186 = 1,284 ft² net
  5. 1,284 × 1.10 = 1,412 ft² with waste
  6. 1,412 ÷ 100 = 14.1 squares, rounded to 14.6 for trim cuts

14.6 squares of siding

Understanding your results

Deducting every opening is slightly generous, because the offcuts from around a window rarely fit anywhere else. Many estimators deduct only openings larger than about thirty square feet for that reason. The waste allowance here is doing that work instead, so do not deduct twice.

Trim is where siding jobs overrun. J-channel around every opening, starter strip along the whole base, corner posts at every external angle and utility trim under the eaves add up quickly, and running short of a trim profile mid-wall means matching a colour batch later.

Questions

About this calculator

Ask something else

One hundred square feet of finished coverage. Siding is quoted and sold that way rather than by the piece, and a box of vinyl typically covers two squares. It is coverage after overlap, not the raw area of the material.

The calculator does, and the waste allowance compensates. If you prefer the traditional approach of deducting only openings over about thirty square feet, drop the window count and reduce the waste allowance to five percent to avoid over-ordering.

Half the width of the gable multiplied by its vertical height from the eaves line to the peak. That treats it as a triangle, which is accurate enough for ordering. Measure each gable separately and add them together before entering the total.

Sometimes, but it is rarely the better choice. Overlaying hides rot and moisture problems, changes how the wall dries, and pushes trim and window reveals out of proportion. Stripping back lets you inspect the sheathing and fit a proper weather barrier.

Keep going

All Roofing & Siding tools
Estimates, not specifications. Results are based on the values you enter and on typical material properties. Densities, coverage rates and product sizes vary between suppliers, so confirm quantities against your supplier's data before ordering. Structural work must be signed off by a qualified professional and meet your local building code.