How to Calculate How Much James Hardie Siding You Need
The formula for calculating James Hardie siding quantity is: measure your total exterior wall area, subtract the area of all windows and doors, then add a waste factor of 10 to 15 percent. Divide that number by the coverage per plank for the width you are installing, and round up to the nearest full box or bundle.
That is the complete calculation. The steps below walk through each part with real numbers so you can apply it to your home before getting quotes or placing a material order.
Perfect Exteriors installs James Hardie siding on homes throughout the Twin Cities metro and greater Minnesota. If you want a professional material takeoff along with an installed price, our James Hardie certified team can provide a free estimate.
Step 1 – Measure Your Total Exterior Wall Area
Measure the height and width of each exterior wall face. Multiply height by width to get the square footage of each wall, then add all wall faces together for a total gross exterior wall area.
For a straightforward two-story home with four rectangular walls, this is four measurements. For homes with gable ends, bump-outs, or dormers, break each irregular shape into rectangles and triangles and calculate each section separately. The area of a triangle is base multiplied by height divided by two – add those to your running total.
One Minnesota-specific measurement note: James Hardie requires a minimum clearance of 6 inches between the bottom of the siding and the ground or grade level. In Minnesota, this matters more than in warmer states because snow accumulation against siding accelerates moisture intrusion into the lowest courses. Start your wall height measurement from 6 inches above grade, not from the ground itself. This reduces your net wall height slightly and is the correct starting point for a material takeoff.
Example: A single-story home with four walls, each 40 feet wide and 9 feet tall (measured from 6 inches above grade to the soffit line), has a gross wall area of 4 x (40 x 9) = 1,440 square feet.
Step 2 – Subtract Windows, Doors, and Other Openings
Measure the height and width of every window, exterior door, garage door, and any other opening that does not require siding. Multiply height by width for each opening and subtract the total from your gross wall area.
For a typical Minnesota home with 10 double-hung windows averaging 3 feet wide by 4 feet tall (12 sq ft each) and two standard entry doors at 3 feet by 6.8 feet (20.4 sq ft each), the total opening area is (10 x 12) + (2 x 20.4) = 120 + 40.8 = 161 sq ft.
Subtract openings from the gross area: 1,440 – 161 = 1,279 square feet of net siding area.
Some estimators skip this step and use the gross area as a conservative buffer, accepting a higher material surplus. That approach works, but it overstates your order by 10 to 15 percent on most homes, which translates to hundreds of dollars in unnecessary material cost on a full re-siding project.
Step 3 – Apply the Waste Factor
No siding installation produces zero waste. Every cut around a window, corner, door, and penetration generates a cut-off piece that cannot be used elsewhere. The standard waste factor for HardiePlank lap siding is 10 percent for a straightforward home with simple geometry and few penetrations. Use 15 percent for homes with multiple gable ends, complex rooflines, or many windows and doors that create a high proportion of short cuts.
Apply the waste factor by multiplying your net siding area by 1.10 or 1.15:
- Simple geometry: 1,279 sq ft x 1.10 = 1,407 sq ft
- Complex geometry: 1,279 sq ft x 1.15 = 1,471 sq ft
This is your adjusted order quantity. Round up to the nearest full box or bundle when placing your order – partial boxes are generally not returnable.
HardiePlank Coverage Reference: Pieces Per 100 Square Feet by Plank Width
HardiePlank lap siding comes in 12-foot lengths in several nominal widths. Each width has a standard exposure (the visible face after overlap), which determines how many planks cover 100 square feet. Use this table to convert your adjusted order quantity into a piece count.
| Plank Width | Standard Exposure | Pieces Per 100 Sq Ft | Notes |
|---|---|---|---|
| 6.25 in. | 5 in. | 20 pieces | Most common for homes matching existing narrow-profile wood siding; traditional cottage or bungalow look |
| 8.25 in. | 7 in. | 15 pieces | Most popular width for full re-siding projects in Minnesota; fewer courses means faster installation |
| 12 in. | 10.75 in. | 10 pieces | Contemporary look with fewer horizontal lines; fewer pieces and courses reduces installation labor time |
Example using the 8.25″ plank at 15 pieces per 100 sq ft:
Adjusted order quantity of 1,407 sq ft divided by 100 = 14.07 squares.
14.07 squares x 15 pieces per square = 211 planks.
Round up to 215 planks (the next full bundle quantity at your supplier).
Using the 6.25″ plank instead increases the piece count to 281 planks for the same wall area – more material cost and more installation time. The wider the plank, the lower the piece count and the faster the install.
Don’t Forget Trim, Corner Boards, and Accessories
HardiePlank lap siding is one component of a complete James Hardie exterior system. The following accessories are required for a code-compliant, warranty-valid installation and need to be included in your material order. The original post omitted this section entirely – it is a common source of material shortfalls on first-time estimates.
HardieTrim Boards
Corner boards, window surrounds, door casings, and frieze boards at the soffit line are all typically installed using HardieTrim fiber cement boards. To estimate trim linear footage, measure the total perimeter of every window and door opening, add the linear footage of all four corners of the building from foundation to soffit, and add the linear footage of any frieze or band boards. A 2,000 square foot home typically requires 300 to 500 linear feet of trim board depending on the number of windows and doors.
Starter Strip
A starter strip is installed at the base of the first siding course to set the correct angle and clearance. James Hardie recommends their own fiber cement starter strip, though metal coil stock is also commonly used. Measure the total linear footage of the base of all exterior walls where siding begins – this equals your starter strip requirement.
Flashing
Z-flashing is required above every horizontal trim board – above windows, doors, band boards, and any horizontal transition. Measure the total linear footage of all horizontal trim locations and specify Z-flashing accordingly. In Minnesota, proper flashing installation at these locations is critical because ice and snowmelt actively seek any gap between trim and siding during freeze-thaw events.
Caulk and Sealant
James Hardie requires paintable, flexible caulk at all butt joints, window and door perimeters, and penetrations. A standard 10-ounce tube covers approximately 30 linear feet at a 3/8-inch bead. Estimate one tube per 150 to 200 square feet of siding as a starting point, adjusted for the number of joints and penetrations on your specific home.
What Affects the Total Installed Cost of James Hardie Siding in Minnesota
Material quantity is only one component of a James Hardie project cost. For Minnesota homeowners planning a full re-siding project, the following factors affect the total installed price beyond the plank count.
- Existing siding removal – removing and disposing of old vinyl, wood, or aluminum siding adds labor and disposal cost; some projects can install Hardie over existing siding where the substrate is sound and flat, which eliminates this cost
- Sheathing and housewrap condition – rotted or damaged sheathing uncovered during demo needs replacement; James Hardie requires a compliant water-resistive barrier behind the siding, which is frequently upgraded during a re-siding project
- ColorPlus vs. primed – ColorPlus factory-finished planks cost more per plank but eliminate the cost of field painting; on a full re-siding project the cost difference often narrows when you factor in paint labor
- Project complexity – multiple stories, steep grades requiring scaffolding, complex trim profiles, and many penetrations all add labor hours
- Time of year – James Hardie fiber cement can be installed in cold weather, but caulk and primer application have temperature requirements; fall installs in Minnesota sometimes require phased completion with final painting deferred to spring
James Hardie siding in Minnesota typically runs $11 to $18 per square foot installed, depending on the above factors. A 2,000 square foot home re-sided with 8.25″ HardiePlank commonly runs $18,000 to $28,000 for a complete project including removal, trim, and a primed finish coat.
Frequently Asked Questions
How many pieces of HardiePlank do I need per square?
It depends on the plank width you are installing. The 6.25-inch plank at a 5-inch exposure requires 20 pieces per square (100 sq ft). The 8.25-inch plank at a 7-inch exposure requires 15 pieces per square. The 12-inch plank at a 10.75-inch exposure requires 10 pieces per square. Always use the net siding area after subtracting openings, then add a 10 to 15 percent waste factor before calculating piece count.
Do I subtract windows and doors when calculating James Hardie siding?
Yes. Measure the width and height of every window, door, and opening, multiply to get square footage, and subtract the total from your gross wall area before applying the waste factor. Skipping this step typically overstates your material order by 10 to 15 percent on a standard home.
What waste factor should I use for James Hardie siding?
Use 10 percent for a home with straightforward rectangular walls, few windows, and simple corners. Use 15 percent for homes with gable ends, multiple dormers, complex architectural details, or a high number of window and door openings. Never use less than 10 percent – fiber cement cannot be field-bent around curved surfaces, and every cut generates a piece that is unlikely to be reusable elsewhere.
Does James Hardie siding need a minimum ground clearance in Minnesota?
Yes. James Hardie installation guidelines require a minimum 6 inches of clearance between the bottom of the siding and finished grade. In Minnesota, where snow accumulation against exterior walls is common for four to five months of the year, maintaining this clearance is more important than in milder climates. Factor this into your wall height measurement when calculating square footage – start from 6 inches above grade, not from the ground.
What is the James Hardie warranty?
HardiePlank lap siding carries a 30-year non-prorated, transferable limited warranty against manufacturing defects. The warranty is non-prorated, meaning James Hardie does not reduce the payout based on the age of the product the way many competitor warranties do. ColorPlus factory-finished products carry an additional 15-year limited warranty on the finish. Installation must be performed by a trained contractor and in compliance with James Hardie’s installation guidelines for the warranty to remain valid. Perfect Exteriors is a James Hardie certified installer.
Can James Hardie siding be installed over existing siding in Minnesota?
In some cases, yes. James Hardie permits installation over existing flat, sound substrates where the installation meets their technical requirements for fastener penetration depth and substrate condition. In practice, most Minnesota re-siding projects involve removal of the existing material because it allows inspection and repair of the housewrap, sheathing, and flashing beneath – all of which frequently need attention on homes 20 or more years old. A Perfect Exteriors consultant can assess whether your existing substrate qualifies for an overlay installation during a free estimate visit.
Get a Professional Material Takeoff for Your Minnesota Home
Calculating James Hardie siding quantity from scratch is straightforward for simple homes, but complex exteriors – multiple stories, many windows, gable ends, board and batten accents – benefit from a professional takeoff that accounts for every profile, trim piece, and accessory in a single accurate order.
Perfect Exteriors provides free estimates for James Hardie siding installations throughout the Twin Cities metro, Monticello, Rogers, Elk River, Coon Rapids, and surrounding communities. Learn more about our James Hardie certified installation services or request a free estimate today.
