How do you calculate Hardie siding?

Find net wall area, add 10% to 15% for waste, then divide by the coverage of one board. A 12 ft board at 7 in exposure covers 7 sq ft, so 1,904 sq ft of required area needs about 272 boards. Round up to whole boards or cartons.

How many square feet does a Hardie plank cover?

It depends on exposure. A 12 ft board covers exactly its exposure in square feet: about 7 sq ft for an 8.25 in board at 7 in exposure, 6 sq ft for a 7.25 in board at 6 in exposure, and 8 sq ft for a 9.25 in board at 8 in exposure.

How many pieces of Hardie siding are in a square?

Divide 100 by the coverage per board. At 7 in exposure a 12 ft board covers 7 sq ft, so a square takes about 14.3 boards, which you round up when ordering. At 6 in exposure it is about 16.7 boards, and at 8 in about 12.5.

What is exposure on lap siding?

Exposure is the visible height of each board once installed. It equals the board width minus the overlap, typically about 1.25 in for fiber cement lap siding. Always confirm the overlap and maximum exposure with the manufacturer's instructions.

How much waste should I allow for fiber cement siding?

A simple rectangular house commonly uses about 10%, and homes with many gables, corners or dormers often need 12% to 15%. Fiber cement is cut to length and offcuts are harder to reuse, so do not go too low.

Do the calculations differ between Hardie and other fiber cement brands?

The method is identical. Only the board widths, exposures and carton or pallet quantities change by brand and product line, so enter your product's numbers. This guide is independent and is not affiliated with or endorsed by James Hardie.

How to Calculate Hardie and Fiber Cement Lap Siding

Fiber cement lap siding, including Hardie®-style plank, is sold as long boards that overlap each course. That makes the math a little different from vinyl: you work from the exposure of each board, the visible strip once it is installed. Get that number right and the rest is division.

This guide is independent and not affiliated with or endorsed by James Hardie. Board sizes and installation rules vary by manufacturer and product line, so always confirm details with the instructions for the product you are buying.

Step 1: Find your net wall area

Add up the area of every wall and gable, then subtract windows, doors and garage doors. If you have not measured yet, follow how to measure for siding. The result is your net area in square feet.

Example: a house with 1,700 sq ft of net wall area.

Step 2: Add waste

Fiber cement is cut to length at corners and openings, and offcuts are less reusable than with some other materials. Many installers plan on 10% for a simple house and 12% to 15% for a complex one. Our fiber cement calculators default to 12%.

1,700 × 1.12 = 1,904 sq ft required.

See siding waste factor for how to pick the right number for your layout.

Step 3: Find the exposure

Lap boards overlap, so only part of each board shows. The exposure is the board width minus the overlap. For fiber cement lap siding the overlap is commonly about 1.25 inches, which gives these typical pairs:

Board width	Typical exposure
5.25 in	4 in
6.25 in	5 in
7.25 in	6 in
8.25 in	7 in
9.25 in	8 in

A board labeled “8 1/4” therefore usually shows about 7 inches. Check the manufacturer’s stated exposure range for your product and pick an exposure within it. Wider exposure means fewer boards, but stay within the allowed range.

Step 4: Calculate coverage per board

Boards are commonly 12 ft long. Multiply length in feet by exposure in inches and divide by 12. Because the length is 12, this simplifies neatly: a 12 ft board covers as many square feet as its exposure in inches.

7 in exposure: 12 × 7 ÷ 12 = 7 sq ft
6 in exposure: 6 sq ft
8 in exposure: 8 sq ft

Step 5: Divide and round up

Divide your required area by coverage per board and round up:

1,904 ÷ 7 = 272 boards.

To see how many boards make a square, divide 100 by the coverage: at 7 in exposure that is about 14.3 boards per square; at 6 in about 16.7; at 8 in about 12.5. The siding panels calculator does this from your own dimensions.

If your supplier sells by the carton, divide by carton coverage instead. Our default is 175 sq ft (25 boards at 7 sq ft each), which you can edit to match your supplier:

1,904 ÷ 175 = 10.9, so order 11 cartons, or 275 boards. Some suppliers sell boards individually or by the pallet, so use the unit they actually quote.

Step 6: Add trim and accessories

Boards are only part of the order. Measure these in linear feet:

Outside and inside corners. See the corner post calculator.
Window and door casings, from the opening perimeters.
Base and starter courses. See the starter strip calculator.
Flashing, joint flashing, fasteners and caulk, following the manufacturer’s installation instructions.
House wrap (house wrap calculator).

Quick reference: whole-house example

Step	Value
Net wall area	1,700 sq ft
Waste at 12%	1,904 sq ft
Board	8.25 in wide, 7 in exposure, 12 ft long
Coverage per board	7 sq ft
Boards needed	272
Cartons at 175 sq ft	11

Cutting and finishing notes

Fiber cement contains crystalline silica, and cutting it creates dust. Manufacturers and safety regulators recommend dust-reducing cutting methods and appropriate respiratory protection, and cutting outdoors where possible. Follow the product’s safety data and installation instructions. Plan on priming or painting cut edges as instructed, and choose between primed boards and factory-finished ones depending on how much on-site painting you want to do.

What does it cost?

Installed, Hardie®-style fiber cement runs roughly $8 to $15 per square foot, or $800 to $1,500 per square, on typical US projects. See Hardie board siding cost and how much siding costs for the factors behind it.

Let the calculator do it

Enter your walls, gables and openings into the Hardie-style siding calculator or the fiber cement siding calculator, adjust board width, exposure and carton size, and it returns boards, cartons and squares with waste included.