Enter the length and height of your walls and any gable ends, then pick the plank width. The calculator gives the number of 12 ft HardiePlank lap siding boards to order, along with squares and linear feet.
How to use the Hardie siding calculator
- Add up the length of every wall you are siding and enter the wall height from the bottom course to the soffit.
- Enter the number of gables with their width and height, or 0 if the roof is hipped.
- Choose the plank width. The calculator uses the exposure, the part left showing once each plank overlaps the one below by 1 1/4 in.
- Keep 10% for waste, or raise it for a house with many corners, windows and short runs.
Hardie siding formulas
Wall area = total wall length × wall height Gable area = number of gables × width × height ÷ 2 Siding area = wall area + gable area − openings One plank covers = 12 ft × exposure (in) ÷ 12 = exposure in ft² Planks = siding area × (1 + waste %) ÷ coverage per plank, rounded up
James Hardie’s installation instructions call for a minimum 1 1/4 in overlap, so an 8 1/4 in plank shows 7 in. A 12 ft plank at 7 in of exposure therefore covers 7 ft².
Worked example: a 40 × 30 ft house, 9 ft walls, two gables
- Walls: (40 + 30 + 40 + 30) × 9 = 140 × 9 = 1,260 ft².
- Gables: 2 × 30 × 7.5 ÷ 2 = 225 ft².
- Siding area: 1,260 + 225 = 1,485 ft².
- 8 1/4 in planks cover 7 ft² each. With 10% waste: 1,485 × 1.10 ÷ 7 = 233.4, or 234 planks.
- That is 16.3 squares and 2,808 linear feet of siding.
HardiePlank widths and coverage
| Plank width | Exposure | One 12 ft plank covers | Planks per 100 ft² (no waste) |
|---|---|---|---|
| 5 1/4 in | 4 in | 4 ft² | 25 |
| 6 1/4 in | 5 in | 5 ft² | 20 |
| 7 1/4 in | 6 in | 6 ft² | 16.7 |
| 7 1/2 in | 6 1/4 in | 6.25 ft² | 16 |
| 8 in | 6 3/4 in | 6.75 ft² | 14.8 |
| 8 1/4 in | 7 in | 7 ft² | 14.3 |
| 9 1/4 in | 8 in | 8 ft² | 12.5 |
| 9 1/2 in | 8 1/4 in | 8.25 ft² | 12.1 |
| 12 in | 10 3/4 in | 10.75 ft² | 9.3 |
Widths and exposures are from the coverage chart in James Hardie’s HardiePlank installation instructions.
How much waste for Hardie siding?
Fiber cement planks come in 12 ft lengths, and every window, door and corner leaves an offcut. Ten percent covers a plain rectangular house. Allow 15% or more for many openings, bay windows or short walls, and do not subtract small windows at all: the cut-outs and the trim around them use up the material you would have saved.
Installation details that affect the order
- Overlap: James Hardie specifies at least 1 1/4 in of overlap between courses, whether you face nail or blind nail.
- Starter strip: the first course sits on a starter strip 1 1/4 in wide, usually ripped from a plank. Add one plank for every 12 ft of wall base if you cut your own.
- Framing: planks are fastened to studs spaced no more than 24 in on center.
- Ground clearance: the instructions call for finished grade to slope away from the house, typically at least 6 in over the first 10 ft, with the siding kept clear of the ground.
Trim, corner boards, flashing and fasteners are ordered separately. Always follow the current installation instructions for your product and region.
Questions
How many Hardie planks do I need for 100 square feet?
With 8 1/4 in planks at 7 in exposure, 100 ÷ 7 = 14.3 planks before waste, or 16 planks with 10% waste.
How many square feet does a Hardie plank cover?
A 12 ft plank covers its exposure in square feet: 4 ft² for 5 1/4 in planks, 7 ft² for 8 1/4 in planks and 10.75 ft² for 12 in planks.
What is the exposure of 8 1/4 Hardie siding?
7 inches. Each plank laps the one below by 1 1/4 in, the minimum James Hardie allows.
How many squares of Hardie siding do I need?
Divide the siding area by 100 and add waste. A house with 1,485 ft² of walls and gables needs about 16.3 squares with 10% waste.
Sources
- HardiePlank Lap Siding Installation Requirements (HZ10), James Hardie, via Florida Building Commission product approval FL13192.
Used for: Plank widths and exposures, 12 ft plank length, 1 1/4 in minimum overlap and starter strip, studs at 24 in on center max, grade sloping 6 in in the first 10 ft. - NIST Guide to the SI, Appendix B.8: Factors for units listed alphabetically, National Institute of Standards and Technology.
Used for: Exact inch and foot definitions; square metre conversions.