Lap Siding Material Calculator
Calculate exact lap siding materials including boards, trim, corner pieces, and fasteners. Our calculator automatically adjusts exposure for even coursing and includes material-specific waste factors per IRC R703 and manufacturer specifications.
Lap Siding Calculator
Industry-standard calculations per IRC R703 & manufacturer specifications
Quick Answer
Most common: 8.25" fiber cement boards at 7" exposure = 15 boards per 100 sq ft. Add 10-15% waste. Calculate even coursing for wall height to avoid uneven top row.
Material Selection
DIY adds 5% waste
Wall Measurements
Wall 1
Openings & Trim
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Want to Learn More?
Even coursing formulas, HardiePlank vs LP SmartSide specs, coverage by exposure width, trim quantities, and waste factors per IRC R703.
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Frequently Asked Questions
What is siding exposure?
Exposure is the vertical measurement of siding visible between overlapping courses — the weather-facing portion. For 8.25-inch HardiePlank, the standard exposure is 7 inches (1.25-inch lap). Maximum allowable exposure is set by the manufacturer; exceeding it voids the warranty and violates IRC R703.3.2. Actual exposure determines how many boards you need: narrower exposure = more boards per wall.
What is even coursing and why does it matter?
Even coursing adjusts the exposure so every horizontal course is identical height from foundation to soffit. Without it, you'd cut the top course to an awkward, uneven height. The formula: adjusted exposure = wall height (inches) ÷ CEILING(wall height ÷ max exposure). Even coursing is a mark of professional installation and is required by James Hardie and LP SmartSide installation specs.
How much overlap does lap siding need?
IRC R703.3.2 requires a minimum 1-inch overlap for wood lap siding. James Hardie (HardiePlank) requires a minimum 1-1/4 inch overlap. LP SmartSide requires 1-inch minimum. Greater overlap reduces effective exposure per board and increases material quantities. Never reduce overlap to stretch coverage — it compromises weather resistance and voids product warranties.
How much waste factor should I add for lap siding?
Standard installations with simple walls need 10% waste for cuts and mis-cuts. Complex walls with many windows, corners, or angles require 15% waste. Fiber cement (HardiePlank, Nichiha) requires an additional 2–5% over engineered wood or vinyl due to brittleness and scoring/snapping waste. Diagonal or pattern installations need 15–20% waste. Always round up to the nearest full board.