Hardie Siding Calculator: Square Feet

This calculator works out the total square footage of HardiePlank or other fiber-cement siding you need for a wall or house, based on the wall’s length and height. You enter the dimensions of each rectangular wall section, and it returns the area in square feet, which is the standard unit for ordering siding. The result helps you estimate how many boards or panels to buy, though you should always add extra for waste and trim.

Formula

The basic formula for siding area is simple: Area = Length × Height for each wall. For multiple walls, add the areas together:

Total Area = (L₁ × H₁) + (L₂ × H₂) + …

Where:

• L = wall length in feet (or meters, if you measure that way)
• H = wall height in feet (or meters)
• Area = square feet (or square meters)

For the calculator above, we multiply length × height × number of walls, assuming all walls are the same size. If your walls differ, calculate each one separately and add them.

Worked Example

Using the default values: a wall that is 20 feet long and 10 feet high, with 4 such walls.

1. Area of one wall: 20 ft × 10 ft = 200 sq ft
2. Total area: 200 sq ft × 4 = 800 sq ft
3. Boards needed: An 8-foot HardiePlank board typically covers about 12 sq ft (8 ft × 1.5 ft exposure). So 800 / 12 = 66.67, rounded up to 67 boards.

The calculator gives 800 sq ft and 67 boards, matching these steps.

Why Square Footage Matters for Hardie Siding

HardiePlank and other fiber-cement siding are sold by the square foot or by the board. Knowing your total square footage is the first step to budgeting and ordering. Most contractors and suppliers ask for this number directly. It also helps you compare quotes, because labor is often priced per square foot (or per “square,” which is 100 sq ft).

Keep in mind that the square footage of the wall is not exactly the same as the siding material needed. You’ll have openings (doors, windows) that you subtract, and you’ll have waste from cutting, plus extra for gables, corners, and trim. A standard practice is to add 10% for waste, but that can go up for complex roofs or many corners.

How to Measure Your Walls Correctly

To get an accurate number, measure each wall from the bottom of the siding (usually the top of the foundation) to the top of the wall (the eave or soffit). Use a tape measure and record in feet and inches. Convert inches to feet by dividing by 12 (e.g., 6 inches = 0.5 ft). For gable ends, measure the base and height, then multiply by 0.5 (area of a triangle).

If you have a two-story house, measure each floor separately because the height might differ. Also measure any attached garage or porch separately. Write down each wall’s dimensions and add them up. A common mistake is to measure the outside of the house but forget that the siding goes up to the soffit, not just to the top of the wall framing.

What to Subtract and What to Add

You don’t need siding over windows and doors, so subtract those areas. For a standard window, measure the width and height and subtract. For a door, subtract the door area (usually 20 sq ft for a typical exterior door). But don’t subtract small openings like vents or light fixtures; the waste from cutting around them is usually more than the area you save.

Add extra for:

• Waste: 10% is typical, but 15% is safer if you have many corners, gables, or a complex roofline.
• Trim and corners: These are separate pieces, not part of the siding area, but you’ll need to order them.
• Starter strips and J-channels: These are also separate.

Common Mistakes When Calculating Siding

The most frequent error is mixing units. If you measure in feet, keep everything in feet. If you measure in inches, convert to feet before multiplying. Another mistake is forgetting to multiply by the number of walls or adding all walls instead of averaging. Also, don’t forget that HardiePlank is installed with a slight overlap (the exposure is less than the board width). For example, a 12-inch-wide plank might have a 10-inch exposure, so the coverage per board is less than you’d think. Always check the manufacturer’s exposure rating.

Finally, don’t round too early. Calculate the total area, then add waste, then round up to the nearest board. Rounding up each wall separately can overestimate.

Hardie Siding vs. Other Materials

Fiber-cement siding like HardiePlank is heavier and more durable than vinyl, but it’s also more expensive. It resists fire, rot, and insects, and it holds paint longer. Vinyl is cheaper and lighter but can crack in cold weather and may look less like wood. Wood siding is natural but requires more maintenance. The square footage calculation is the same for all, but waste factors and board sizes differ. For fiber-cement, boards are typically 8 to 12 feet long and 8.25 to 12 inches wide, with exposure varying.

Cost Considerations

While this calculator gives square footage, it doesn’t give cost because prices vary by region, product line, and labor rates. However, you can use the square footage to get quotes. As of recent data, fiber-cement siding material costs roughly $1 to $3 per square foot, with installation adding another $2 to $5 per square foot. But these are rough figures; always get local quotes. Also factor in painting, which is often needed after installation.

Frequently Asked Questions

How many square feet does one HardiePlank board cover?

It depends on the board size and exposure. A common 8-foot board with 10-inch exposure covers about 6.67 sq ft (8 ft × 0.833 ft). A 12-foot board with 10-inch exposure covers 10 sq ft. Always check the product specs, but for rough estimates, you can use 12 sq ft per 8-foot board if the exposure is 12 inches (which is rare).

Do I subtract windows and doors from the square footage?

Yes, you should subtract the area of large openings like windows and doors to avoid over-ordering. But keep in mind that cutting around openings creates waste, so don’t subtract every small opening. A good rule is to subtract windows and doors, then add 10-15% waste.

What is a “square” in siding terms?

A “square” is a unit of area equal to 100 square feet. Siding is often priced and sold by the square. To convert your total square footage to squares, divide by 100. For example, 800 sq ft is 8 squares.

How much extra should I order for waste?

Typically 10% is standard, but for roofs with many angles, gables, or a complex layout, order 15-20% extra. It’s better to have a few extra boards than to run out mid-project, because matching the color after a reorder can be difficult.

Can I use this calculator for gable ends?

Yes, but you need to measure the base and height of the triangle and use the formula: Area = 0.5 × base × height. Then add that to the rectangular wall areas. The calculator above assumes rectangles, so for gables, do the triangle separately.

Does the calculator account for the overlap of HardiePlank?

No, the calculator gives the wall area, not the actual board coverage. Because planks overlap, the exposed surface is less than the full board width. To get the true number of boards, divide the wall area by the exposed coverage per board, not the full board width.

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in l | Wall length | ft | 20 | 0
in h | Wall height | ft | 10 | 0
in n | Number of walls | - | 4 | 1
out area | Total wall area | sq ft | l * h * n | 0
out boards | Boards needed (8 ft length) | boards | ceil((l * h * n) / (8 * 1.5)) | 0