Lap Siding Calculator: HardiePlank, James Hardie & LP SmartSide
How many HardiePlank boards do you need? This free Hardie siding calculator works for fiber cement (HardiePlank, James Hardie, Nichiha), engineered wood (LP SmartSide), wood bevel and clapboard, and vinyl lap siding. Homeowners and contractors get an instant material list — boards per square, trim, corners, fasteners, and flashing — with even coursing automatically calculated.
Lap siding mistakes show up forever. Uneven coursing leaves a narrow strip at the top of every wall — visible from the street for the life of the home — and missing the 1.25" minimum overlap on fiber cement voids the James Hardie warranty entirely. Most siding shortages come from forgetting trim and corner boards.
Auto-adjusts exposure for even coursing per IRC R703 and includes manufacturer-specific waste factors.
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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Related Code Guides
Building code, climate zone, and standards references that change defaults for this calculator.
Climate Zone 1: R-Value Requirements (2021 IECC)
Climate Zone 1 R-value minimums for ceilings, walls, floors, and slabs in Florida Keys, Hawaii, and southern Texas under the 2021 IECC.
Florida HVHZ Roofing Requirements
Florida HVHZ wind-zone roofing rules: shingle ratings, fastener schedules, NOA approval, and underlayment per FBC 8th Edition (2023) and Miami-Dade County.
Texas Windstorm Roofing Requirements
Texas Department of Insurance Seacoast windstorm rules: shingle ratings, fastening, WPI-8 inspection, and Tier I/II requirements for the Texas coast.
Frequently Asked Questions
How do I use this as a HardiePlank or James Hardie siding calculator?
Select 'Fiber cement (HardiePlank)' as the material. The calculator applies James Hardie's required 1¼" minimum overlap (vs. 1" for wood and LP SmartSide), uses HardiePlank's standard plank widths (5¼", 6¼", 7¼", 8¼", 9¼", 12"), and includes the 2–5% extra waste factor specific to fiber cement (brittleness, scoring/snapping cuts). Outputs include boards per square (10 ft × 10 ft = 100 sq ft), trim (HardieTrim), corner boards, and stainless or hot-dip galvanized fasteners per James Hardie HZ5/HZ10 spec.
How many pieces of 8.25" HardiePlank are in a square (100 sq ft)?
8¼" HardiePlank installed at the standard 7" exposure (1¼" overlap) covers 7" × 12 ft = 84 in × 144 in = 12,096 in² per board ÷ 144 = 84 sq ft per LF of plank length, or 7 sq ft per LF of board × 12 ft length = 84 sq ft per board (gross). Net coverage with overlap is 12 ft × 7" / 12 = 7 sq ft per board. So a square (100 sq ft) takes about 14.3 boards before waste, or 16 boards with the 12% recommended waste factor for fiber cement. The calculator handles the even-coursing adjustment automatically — your actual board count depends on the wall heights you enter.
Can I use this as an LP SmartSide calculator?
Yes. Select 'Engineered wood (LP SmartSide)' as the material. The calculator applies LP's 1" minimum overlap requirement, uses standard SmartSide widths (6", 8", 12", 16"), and applies the lower waste factor LP allows for engineered wood (10% simple walls, 15% complex). LP SmartSide is lighter and easier to cut than fiber cement so the waste premium fiber cement requires doesn't apply. Fastener output is sized for the LP-specified ring-shank or screw schedule rather than the heavier-gauge fiber cement nails.
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.