Hardie Siding Calculator
Calculate how many James Hardie fiber cement lap siding planks you need based on wall area and your chosen exposure width.
Planks needed
170planks
Coverage per plank
7ft²
Net wall area
1077ft²
Exposure is the visible face of each plank after overlap — a narrower exposure looks more traditional but needs more planks per square foot. Check your specific product's installation guide.
How the hardie siding calculator works
Fiber cement lap siding coverage depends on the plank's length and how much of it stays visible (exposure) after the next row overlaps it. Multiply those two numbers to get how much wall area one plank actually covers, then divide your total wall area by that.
Formula: coverage per plank (ft²) = plank length (ft) × (exposure (in) ÷ 12). Net wall area = perimeter × height − openings. Planks = (net area × (1 + waste% ÷ 100)) ÷ coverage per plank, rounded up.
Frequently asked questions
Exposure is the visible portion of each plank once it's installed and overlapped by the plank above it — it's less than the plank's full width. A 7-inch exposure means each row shows 7 inches of the board's face to the weather, even though the board itself is wider.
12 feet is the standard length for HardiePlank lap siding, which is why it's the default here. Confirm your specific product line, since some styles and regions carry different lengths.
A narrower exposure (more overlap) shows less of each board per row, so it takes more boards to cover the same wall area. A wider exposure covers more area per board but shows a bolder, less traditional reveal.
Fiber cement lap siding is sold and installed as discrete planks with a fixed length and chosen exposure, so it's counted by the piece. The general siding calculator works in squares (100 sq ft units), which fits panel and shake products better.