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Gross wall area (all sides)
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sq ft
Total area of openings to subtract
Enter openings area (0 or more)
Result
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How to Calculate HardiePlank Siding
James Hardie fiber cement siding (HardiePlank) is measured and sold by the plank, box, or roofing square (100 sq ft). To estimate your project you need the net wall area (gross area minus windows and doors), the plank exposure (the visible portion of each plank), and a waste factor for cuts.
The most popular HardiePlank product is the 8.25-inch wide lap plank installed with a 7-inch exposure. A 12-foot plank at 7-inch exposure covers 7.0 square feet. Divide your total net area by coverage per plank to find the number of planks needed.
Formula
Planks = (Net Wall Area × Waste Factor) ÷ Coverage per Plank
Coverage per plank = (Exposure inches ÷ 12) × plank length in feet. Example: 8.25" plank with 7" exposure on 12ft plank = (7/12) × 12 = 7.0 sq ft per plank. 1,800 sq ft wall × 1.10 waste ÷ 7.0 = 283 planks = 48 boxes.
💡 Pro tip: Order all planks from the same production run (same dye lot) to ensure consistent colour. Hardie siding expands slightly — follow installation instructions for nail placement and gap spacing to prevent buckling or cracking.
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Frequently Asked Questions
Measure total gross wall area, subtract openings (windows and doors), multiply by your waste factor (10%), then divide by the coverage per plank based on exposure width. This calculator does all of that for you.
The most common is 7-inch exposure on an 8.25-inch plank. Wider exposures show more of each board and install faster but require care on windy facades. Follow Hardie's installation guide for minimum overlaps.
8.25-inch HardiePlank 12-ft comes approximately 6 planks per box. 7.25-inch boards come 8 per box. Always verify on your specific product packaging as counts can change.
Material cost: $0.80–$1.50 per square foot (2024). Installed cost including labor, trim, and accessories: $6–$12 per square foot. Regional pricing varies significantly.
Measure each opening (width x height) and subtract from gross wall area. As a quick rule of thumb: deduct 15 sq ft per standard window and 20 sq ft per door.
James Hardie carries a 30-year limited transferable warranty and realistically lasts 50+ years. It resists fire, termites, rot, and moisture far better than wood or vinyl. The paint finish warranty is typically 15 years — repainting every 10–15 years is the main maintenance task. Properly installed Hardie is one of the most durable residential cladding options available.