Calculators · Siding Calculator

Walls & Finishes

Siding Calculator

Add your walls and gables, subtract windows and doors, and get the siding area in square feet, siding squares and lap siding pieces.

Wall 1

Wall length

Wall height

Number of identical walls

Gable 1

Gable base

Gable height (base to peak)

Number of identical gables

Opening 1

Opening width

Opening height

Number of identical openings

Opening 2

Opening width

Opening height

Number of identical openings

Waste to add (%)

Optional. Empty means 0. See the waste guide below.

Lap siding pieces (optional)

Pick a James Hardie lap width to fill in its exposure, or type your own below.

Exposure (inches)

The visible part of each piece. Empty means no piece count.

Length of each piece

James Hardie planks in its charts are 12 ft.

How to calculate how much siding you need

Siding is measured by the area of wall it covers. Work out the area of each wall, add the triangular gables, then take away the openings that will not get siding:

Step	Formula
Rectangular wall	length × height
Triangular gable	base × height ÷ 2
Door, window or garage door	width × height
Net wall area	walls + gables − openings
Siding squares	net wall area ÷ 100
Area to order	net wall area × (1 + waste ÷ 100)

Measure the gable height from the top of the wall, where the triangle starts, up to the peak. Walls that are the same size can share one row: type the number of identical walls in the last box. The same goes for windows that are the same size. Each measurement accepts feet and inches written as 9'6", and you can switch any box to inches, yards or meters.

Example: a house with four walls and two gables

A house has four walls of 30 ft by 9 ft and two gables with a base of 30 ft and a height of 8 ft. It has 12 windows of 3 ft by 4 ft and one door of 3 ft by 7 ft. This is what the calculator does with those numbers:

Step	Operation	Result
Walls	4 × 30 × 9	1,080 sq ft
Gables	2 × (30 × 8 ÷ 2)	240 sq ft
Walls and gables	1,080 + 240	1,320 sq ft
Windows	12 × (3 × 4)	− 144 sq ft
Door	1 × (3 × 7)	− 21 sq ft
Net wall area	1,320 − 144 − 21	1,155 sq ft
Siding squares	1,155 ÷ 100	11.55

Wall area by perimeter and wall height

If you only know the outline of a house, multiply the distance around it by the wall height. This is the area of the walls alone. It does not include gables, and you still need to subtract the openings.

Perimeter	8 ft walls	9 ft walls	10 ft walls
120 ft	960 sq ft	1,080 sq ft	1,200 sq ft
140 ft	1,120 sq ft	1,260 sq ft	1,400 sq ft
160 ft	1,280 sq ft	1,440 sq ft	1,600 sq ft
200 ft	1,600 sq ft	1,800 sq ft	2,000 sq ft

Lap siding exposure and pieces per square

Lap siding overlaps from one course to the next, so each piece covers only its exposure, the part you see on the wall. A piece covers its exposure divided by 12, times its length. These are the lap widths and exposures that James Hardie lists in the coverage charts of its installation guides, with the area a 12 ft piece covers and the pieces needed for one square. We checked them in October 2026.

Lap width	Exposure	Area per 12 ft piece	Pieces per square
5-1/4 in.	4 in.	4.00 sq ft	25.00
6-1/4 in.	5 in.	5.00 sq ft	20.00
7-1/4 in.	6 in.	6.00 sq ft	16.67
7-1/2 in.	6-1/4 in.	6.25 sq ft	16.00
8 in.	6-3/4 in.	6.75 sq ft	14.81
8-1/4 in.	7 in.	7.00 sq ft	14.29
9-1/4 in.	8 in.	8.00 sq ft	12.50
9-1/2 in.	8-1/4 in.	8.25 sq ft	12.12
12 in.	10-3/4 in.	10.75 sq ft	9.30

Sources: HardiePlank installation instructions (HS11119, 05/16) and Hardie Artisan lap siding installation instructions (ASP1725, 02/23). Both guides list planks of 12 ft. In the 2016 chart the columns for the 9-1/4 and 9-1/2 inch widths show the same plank counts, so we only take the exposure from that chart and calculate the pieces ourselves. Product lines change, so check that the width you want is still sold. If you use another brand, type the exposure and length printed on its package.

This calculator rounds the piece count up, because you cannot buy part of a piece. As a check, Hardie Artisan’s chart, which includes a 5% waste factor, shows 18 planks for one square at a 6 inch exposure and 15 planks at a 7 inch exposure. Enter 100 sq ft of wall with 5% waste in this calculator and you get the same two numbers.

How much waste to add

Leave the waste box empty to see the exact area. For orientation, James Hardie shows a 5% waste factor in the coverage chart of its Artisan lap siding guide. The chart in its HardiePlank guide says it does not include waste. James Hardie also states that the charts are a guide and that actual usage depends on variables such as the building design, and it does not take responsibility for ordering too much or too little. A house with many gables, corners and small openings uses more material than a plain box, so confirm the figure with your supplier or installer. Figures checked in October 2026.

What this calculator does not do

It gives the wall area, the siding squares and, if you choose an exposure, the number of pieces of lap siding. It does not give prices and it does not count corner posts, J-channel, starter strips, trim, flashing, house wrap or fasteners. Those are measured in linear feet or by the piece, so take their lengths from the corners and openings of your house and ask your supplier for the coverage of each product. It does not give exposures for vinyl, wood or other brands, because we could not verify those figures in an open manufacturer document. For those, type the exposure and length of your product.

Frequently asked questions

How do you calculate how much siding you need?

Add up the area of every wall (length × height) and every gable (base × height ÷ 2), then subtract every window, door and garage door (width × height). The result is the net wall area in square feet. Divide it by 100 to get siding squares. For example, a house with 4 walls, 2 gables and 13 openings can come to a net area of 1,155 square feet, which is 11.55 squares.

How many square feet are in a square of siding?

One square of siding covers 100 square feet of wall. James Hardie uses that definition in the coverage charts of its installation guides. A wall area of 1,155 square feet is 11.55 squares.

How much siding do I need for a 1,200 sq ft house?

The floor area alone does not tell you, because siding goes on the walls. What counts is the perimeter and the wall height. As an example, a single-story house with a 40 ft by 30 ft footprint (1,200 square feet) and 9 ft walls has a perimeter of 140 ft, so the walls are 140 × 9 = 1,260 square feet before you add gables and subtract openings. Measure your own walls and type them in.

How much does siding cost?

This calculator does not give prices, because they change by product, region and season. Multiply the squares it gives you by the price per square that your supplier quotes, and ask whether that price includes trim, corners and fasteners.

How much waste should I add to a siding order?

The published guidance varies by product, so ask your supplier or the manufacturer. James Hardie shows a 5% waste factor in the coverage chart for its Artisan lap siding, while the chart in its HardiePlank guide says it does not include waste. James Hardie adds that its charts are a guide and that real usage depends on the building design. Type the figure you choose in the waste box, or leave it empty to see the exact area.

How do I work out how many pieces of lap siding I need?

Divide the area by the area one piece covers. That area is the exposure (the visible part of each piece, in inches) divided by 12, times the length of the piece in feet. A 12 ft piece with a 7 inch exposure covers 7 square feet, so 1,155 square feet needs 165 pieces before waste. Pick a James Hardie lap width in the calculator to fill in its exposure, or type the exposure printed on your product.

Does this calculator include corners, J-channel and trim?

No. Corner posts, J-channel, starter strips, window and door trim, flashing and fasteners are sold by the piece or by the linear foot, and they depend on how many corners and openings the house has. Measure them separately and ask your supplier for the coverage of each product.