Hardie Siding Calculator

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Hardie Siding Coverage Calculator

Estimate fiber cement planks, area, and waste for your siding project.

Sum of all exterior wall lengths
Distance from foundation to eaves
Avg. 15 sq.ft each (subtracted)
Avg. 20 sq.ft each (subtracted)
8.25″ (7″ Exposure) 7.25″ (6″ Exposure) 6.25″ (5″ Exposure) 5.25″ (4″ Exposure)
Usually 10% – 15% for cuts

Estimation Results

Net Wall Area
0 sq.ft
Total Square Footage (inc. Waste)
0 sq.ft
12′ Boards Required
0
Square (100 sq.ft units)
0

*Estimation based on standard 1.25″ overlap. 12-foot board lengths assumed.

Understanding Hardie Siding Measurements

James Hardie fiber cement siding, commonly known as Hardie Board or Hardie Plank, is measured by the "Square." In the siding industry, one square equals 100 square feet of wall coverage. When calculating Hardie siding, you must account for the overlap (exposure) rather than the full width of the board.

How to Use the Hardie Siding Calculator

To get an accurate estimate for your home renovation or new construction project, follow these steps:

  • Measure Perimeter: Measure the total linear length of all walls to be sided.
  • Measure Height: Measure from the bottom of the first course to the top of the wall. For gables, use the average height or calculate triangles separately.
  • Deduct Openings: Standard doors are roughly 20 sq.ft, and windows average 15 sq.ft. Subtracting these prevents over-ordering.
  • Exposure Logic: An 8.25″ Hardie Plank has a 7″ exposure because of the 1.25″ overlap required for weatherproofing.

Coverage Table (per 12-foot board)

Plank Width Exposure Sq. Ft. per Board
8.25 inches 7.0 inches 7.00
7.25 inches 6.0 inches 6.00
6.25 inches 5.0 inches 5.00

Why Fiber Cement Requires a Waste Factor

Unlike vinyl siding, fiber cement is heavy and brittle. During installation, boards may crack or requires complex cuts around gables, soffits, and trim. A 10% waste factor is standard for rectangular walls, while 15% to 20% is recommended for homes with many gables or complex architectural features.

Example Calculation

Imagine a wall 40 feet long and 10 feet high:

  1. Gross Area: 40 x 10 = 400 sq.ft.
  2. Subtract 2 Windows: 400 – (2 x 15) = 370 sq.ft.
  3. Add 10% Waste: 370 x 1.10 = 407 sq.ft.
  4. Calculate Boards (8.25″ width): 407 / 7.0 = 59 boards needed.
function calculateHardie() { // Inputs var length = parseFloat(document.getElementById('totalLength').value); var height = parseFloat(document.getElementById('avgHeight').value); var windows = parseFloat(document.getElementById('numWindows').value); var doors = parseFloat(document.getElementById('numDoors').value); var waste = parseFloat(document.getElementById('wasteFactor').value); var plankW = parseFloat(document.getElementById('plankWidth').value); // Validation if (isNaN(length) || isNaN(height) || length <= 0 || height <= 0) { alert("Please enter valid positive dimensions for length and height."); return; } // Logic var grossArea = length * height; var deduction = (windows * 15) + (doors * 20); var netArea = grossArea – deduction; if (netArea < 0) netArea = 0; var totalAreaWithWaste = netArea * (1 + (waste / 100)); // Exposure is plank width – 1.25 inches var exposureInches = plankW – 1.25; // Coverage per 12ft board: 12ft * (exposure / 12) = exposure in feet equivalent var sqftPerBoard = exposureInches; // Simplified because 12 * (exp/12) = exp var boardCount = Math.ceil(totalAreaWithWaste / sqftPerBoard); var squares = (totalAreaWithWaste / 100).toFixed(2); // Display document.getElementById('resNetArea').innerText = Math.round(netArea).toLocaleString() + " sq.ft"; document.getElementById('resTotalArea').innerText = Math.round(totalAreaWithWaste).toLocaleString() + " sq.ft"; document.getElementById('resBoardCount').innerText = boardCount.toLocaleString(); document.getElementById('resSquares').innerText = squares; document.getElementById('resultsArea').style.display = 'block'; // Smooth scroll to results document.getElementById('resultsArea').scrollIntoView({ behavior: 'smooth', block: 'nearest' }); }

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