What is a square of siding?

One square covers 100 square feet of wall. It is the trade unit siding is ordered, quoted, and priced in, which is why converting your wall area into squares is the last step of any siding estimate — divide total adjusted square footage by 100. A modest ranch typically lands around 9 to 12 squares; a larger two-story house with gables can run 20 or more.

Do you subtract windows and doors when calculating siding?

Yes, using rough-opening areas. Standard figures are about 21 square feet for a 3-by-7-foot entry door and 15 square feet for a 3-by-5-foot window. On a house with a dozen windows that adds up to a meaningful deduction — around 180 square feet, nearly two full squares. Override the standard figures for oversized units: a large picture window or a sliding patio door is several times a standard window and skipping that correction over-orders noticeably.

How do you calculate gable area for siding?

A gable is a triangle, so its area is half the base times the height — half the wall width times the rise from the eave to the peak. Add each gable, plus any dormer faces, as extra area on top of perimeter times wall height, because the simple perimeter-by-height rectangle misses them entirely. On a house with two large gables this is easily a couple of squares, and it is the most commonly forgotten part of a siding takeoff.

What waste factor should you use for siding?

Ten percent is a reasonable default for straightforward lap siding on simple rectangular walls. Push toward 15 percent when the walls are cut up with many windows and doors, when there are multiple gables and dormers requiring angled cuts, or when the pattern has a repeat you must maintain across seams. Very long uninterrupted runs waste less; complex elevations waste considerably more, because every angled cut generates an offcut you cannot use elsewhere.

Should you order extra siding for future repairs?

It is worth considering, particularly for vinyl. Colours are discontinued and, more importantly, existing siding fades — so a replacement panel bought in five years will not match even if the same product is still sold. Setting aside a few spare pieces from the original run means a future impact repair is invisible rather than obvious. Store them flat, out of sunlight, somewhere you will remember.

How Much Siding Do You Need? Sizing Guide

Perimeter × height, plus gables, minus openings, plus waste, divided by 100. Here's each term — with worked ranch and two-story examples.

Siding is sold in squares — one square covers 100square feet of wall — so every estimate ends by converting wall area into that unit. Two things trip people up: gables get forgotten (a plain perimeter × height rectangle misses them entirely, and they are often a couple of squares), and waste gets under-budgeted on cut-up elevations where every angled cut produces an unusable offcut. The Siding Calculator runs the method below.

The Formula

Squares = ((perimeter × height + extra area − openings) × (1 + waste)) ÷ 100.

1. Perimeter × average wall height

Measure around the walls being sided and multiply by average wall height. This gives the rectangular bulk of the job. If different elevations have different heights, work them separately and add rather than averaging aggressively.

2. Extra area — gables and dormers

The step most often skipped. A gable is a triangle: area = ½ × base × height, where the base is the wall width and the height is the rise from eave to peak. Add each gable and each dormer face. On a house with two decent gables this is easily a couple of squares that a perimeter-only measurement misses entirely.

3. Opening deductions

Standard rough-opening areas: 21 sq ft per door (a 3×7 ft entry) and 15 sq ft per window (a 3×5 ft unit). A dozen windows is about 180 sq ft — nearly two squares, so it is worth deducting.

Override these for oversized units. A sliding patio door or a big picture window is several times a standard opening; treating it as 15 sq ft over-orders. Count it separately and add its real area to the deduction.

4. Waste factor

10% for straightforward lap siding on simple rectangular walls. Move toward 15% when the elevations are cut up with many openings, when there are multiple gables and dormers needing angled cuts, or when a pattern repeat has to be maintained across seams. Long uninterrupted runs waste less; complex houses waste more, because angled offcuts rarely fit anywhere else.

Worked Example: Simple Ranch

120 ft perimeter, 8 ft walls, no gable area, 1 door, 8 windows, 10% waste:

Gross wall: 120 × 8 = 960 sq ft
Openings: (1 × 21) + (8 × 15) = 21 + 120 = 141 sq ft
Net wall: 960 − 141 = 819 sq ft
Plus 10% waste: 819 × 1.10 = 900.9 sq ft
Squares: 900.9 ÷ 100 = 9.01 squares → order 10

Worked Example: Two-Story with Gables

140 ft perimeter, 9 ft average walls, 120 sq ft of gable area, 2 doors, 12 windows, 10% waste:

Gross wall: (140 × 9) + 120 gable = 1,260 + 120 = 1,380 sq ft
Openings: (2 × 21) + (12 × 15) = 42 + 180 = 222 sq ft
Net wall: 1,380 − 222 = 1,158 sq ft
Plus 10% waste: 1,158 × 1.10 = 1,273.8 sq ft
Squares: 1,273.8 ÷ 100 = 12.74 squares → order 13

Note the gable contribution: those 120 square feet are 1.2 squares on their own. Omit them and you are more than a full square short, which on a re-side means stopping work to reorder and hoping the dye lot matches.

Ordering Tips

Round up to whole squares. Siding is sold that way, and being short mid-job is far more expensive than a modest surplus.
Order all of it at once. Colour varies between production runs. A second order weeks later may not match the first, especially in vinyl.
Keep a few spares. Existing siding fades, so a replacement bought in five years will not match even if the product still exists. Store spares flat and out of sunlight.
Count trim separately. Corner posts, J-channel, and starter strip are ordered by linear foot, not by square — see the installation guide for what you need.

Related Guides

How to Install Lap Siding — Flashing, starter strip, exposure, and fastening
Siding Types Compared — Pick the material before you price the squares
What Drives Siding Costs — Turning squares into a budget

Related Calculators

Siding Calculator — Runs this method with adjustable openings and waste
Insulation Calculator — While the walls are open
Gutter Calculator — Usually replaced during a re-side

Written by Daniel McCarney — AceCalc

How Much Siding to Order