To figure siding, add up each wall (width × height) and each gable (½ × base × height), subtract large openings, divide by 100 to get squares and add 10% for waste. Lap siding planks per square = 100 ÷ (exposure in feet × plank length): with 12 ft HardiePlank at 7 in exposure that is 14.3, so about 14 planks per square.
This siding calculator is for homeowners pricing a re-side, DIYers ordering fiber-cement or vinyl lap siding, and anyone checking a contractor's takeoff. It returns squares, planks for any width and exposure, starter strip, J-channel and corner trim lengths, nails for vinyl, and the cost to side a house at an editable price per square.
How to use the siding estimator
- Walk around the house and enter each wall as width, height in feet, one per line. Measure height from the bottom of the siding to the soffit.
- Enter each gable end as base, peak height. The base is the wall width; the height runs from the eave line to the peak.
- List windows and doors as width, height. Choose whether to deduct them; either way they set the J-channel length.
- Pick a plank width. The exposure fills in from James Hardie's chart, or type your own exposure for vinyl, wood or another brand.
- Enter corner counts and height for corner posts, then your installed price per square.
The formulas and where they come from
The Vinyl Siding Institute's installation manual breaks every house into rectangles and triangles: wall area = height × width, gable area = ½ × height × width, and every 100 sq ft is a square. It says windows and doors usually aren't deducted, large ones like garage and sliding doors can be, and its estimating worksheet ends with “add a factor of 10 percent to all material estimates to allow for waste.” It also sizes starter strip as the linear feet around the base of the house and fasteners as square feet × 0.005 lb for aluminum nails or × 0.01 lb for roofing nails, staples and screws.
For plank counts, James Hardie's HZ5 installation instructions sell HardiePlank in 12 ft lengths with a minimum 1 1/4 in overlap, and publish a coverage chart of 12 ft planks per square that “does not include waste.” Coverage per plank = exposure ÷ 12 × length, and planks = area ÷ coverage per plank, rounded up.
HardiePlank planks per square by width and exposure
Computed from 100 ÷ (exposure ÷ 12 × 12 ft); the whole-number column matches James Hardie's chart. It is the answer to “how many HZ5 planks per square” for each width.
| Plank width | Exposure | Sq ft per 12 ft plank | Planks per square | Chart (1 sq) | 10 squares | 20 squares |
|---|---|---|---|---|---|---|
| 5 1/4 in | 4 in | 4 | 25 | 25 | 250 | 500 |
| 6 1/4 in | 5 in | 5 | 20 | 20 | 200 | 400 |
| 7 1/4 in | 6 in | 6 | 16.67 | 17 | 167 | 333 |
| 7 1/2 in | 6 1/4 in | 6.25 | 16 | 16 | 160 | 320 |
| 8 in | 6 3/4 in | 6.75 | 14.81 | 15 | 148 | 296 |
| 8 1/4 in | 7 in | 7 | 14.29 | 14 | 143 | 286 |
| 9 1/4 in | 8 in | 8 | 12.5 | 13 | 125 | 250 |
| 12 in | 10 3/4 in | 10.75 | 9.3 | 9 | 93 | 186 |
The chart also lists a 9 1/2 in plank at 8 1/4 in exposure, but its counts repeat the 9 1/4 in column, so we left it out; use the “Other” width and enter your exposure.
How many HZ5 7.25 in planks per square?
A 7 1/4 in HardiePlank installs at a 6 in exposure, so each 12 ft board covers 6 sq ft and one square needs 16.67 planks, which James Hardie's chart rounds to 17. With 10% waste that is 18.3 per square. The popular 8 1/4 in plank at 7 in exposure covers 7 sq ft per board, 14.3 per square.
Cost to side a house
The Remodeling Cost vs Value Report prices a standard job: “Replace 1,250 square feet of existing siding” with factory primed and painted fiber cement, or with vinyl including factory trim at openings and corners. James Hardie cites the 2025 report as saying fiber cement adds $24,420 of resale value, a 113.7% return, which implies about $21,478 for the job, or $1,718 per square. Consumer Reports, citing the same report, says the average vinyl job costs nearly $18,000, roughly $1,440 per square. We could not read the report's own national table, so treat these as benchmarks and replace them with a local quote.
| Net siding area | Squares | Squares with 10% waste |
|---|---|---|
| 800 sq ft | 8 | 8.8 |
| 1,000 sq ft | 10 | 11 |
| 1,250 sq ft | 12.5 | 13.8 |
| 1,500 sq ft | 15 | 16.5 |
| 1,800 sq ft | 18 | 19.8 |
| 2,000 sq ft | 20 | 22 |
| 2,500 sq ft | 25 | 27.5 |
| 3,000 sq ft | 30 | 33 |
To price a job, multiply the squares you order by your contractor's installed price per square. Remodeling 2025 Cost vs. Value Report data is excerpted here as narrative only; see www.costvsvalue.com.
Cost is applied to the net area, not the order quantity, because the benchmark is an installed job price that already includes the contractor's waste.
Worked examples
Single-story ranch with two gables
A 40 × 30 ft footprint with 9 ft walls has 2 × 40 × 9 + 2 × 30 × 9 = 1,260 sq ft of wall. Two 30 ft gables 8 ft high add 2 × ½ × 30 × 8 = 240 sq ft. Deducting eight 3 × 4 ft windows and two 3 × 6.75 ft doors (136.5 sq ft) leaves 1,364 sq ft. With 10% waste: 1,500 sq ft = 15 squares, or 215 HardiePlank 8 1/4 in boards (250 at 7 1/4 in). Starter strip is the 140 ft perimeter, J-channel 151 lin ft, and four 9 ft outside corners need 36 lin ft of corner trim.
Two-story box with a hip roof
50 × 30 ft with 18 ft walls is 2,880 sq ft. Twenty 3 × 4 ft windows remove 240 sq ft, leaving 2,640 sq ft. Plus 10% is 2,904 sq ft, 29.04 squares, and 415 planks at 7 in exposure. Vinyl nails at the VSI rate: 14.5 lb aluminum or 29 lb roofing nails.
One gable end only
Re-siding a 28 ft gable that rises 7 ft: ½ × 28 × 7 = 98 sq ft. With 10% waste that is 107.8 sq ft, 16 planks at 7 in exposure. Triangles waste more than rectangles because every course is cut at the rake, so consider more than 10% here.
Lap siding: exposure and overlap
Lap siding is sold by plank width but covers by exposure, the part left visible after the next course overlaps it. James Hardie requires at least 1 1/4 in of overlap, which is why every width in its chart has an exposure 1 1/4 in smaller (8 1/4 in shows 7 in). For vinyl, wood or engineered-wood lap siding, enter the exposure and length from that product's instructions; the per-square math is the same.
Measuring a house for siding
Work around the house one side at a time and sketch each elevation. Measure the width of each wall at the base and its height from the lowest course of siding to the underside of the soffit. On a two-story wall, measure the full height in one go rather than adding floors, so you don't miss the band at the floor line. Bump-outs, chimneys and porches are just more rectangles: list each face as its own line. Skip any wall that will get brick, stone or another material.
Gables are triangles above the eave line. Measure the base (the wall width at the eave) and the rise from the eave to the peak; the calculator halves base × rise. A gambrel or a dormer is a mix: the VSI manual treats gambrel ends as a trapezoid plus triangle and dormer sides as triangles, which you can enter as rectangles and gable triangles.
Finally, count corners. Outside corners get corner posts or corner boards; inside corners get an inside post or J-channel. Enter the count and the height of the tallest wall, and the calculator multiplies them for the trim length.
Common mistakes
- Using plank width instead of exposure. 8 1/4 in planks cover 7 in each; using 8.25 undercounts by about 15%.
- Forgetting gables and dormers. A 30 ft gable 8 ft high is 120 sq ft, more than a square.
- Double-deducting. If you skip deducting windows (VSI method), don't also cut the waste factor.
- Measuring to the roof line. Siding stops at the soffit; measure the wall that will actually be covered.
- Treating the benchmark as a quote. Cost vs Value is a national average for a defined job. Tear-off, rot repair, height and region change the price.
Painting the siding later? Enter the net area in the paint calculator with your exterior paint's coverage. For the roof, use the roofing shingles calculator; for trim boards, the linear feet calculator; and for the overall budget, the renovation cost calculator.
Frequently asked questions
How do I figure siding for a house?
Measure each wall's width × height, add each gable as ½ × base × height, subtract large openings, then divide by 100 to get squares and add 10% for waste. A 40 × 30 ft ranch with 9 ft walls and two 30 ft gables 8 ft high has 1,500 sq ft before openings, 1,364 sq ft after ten openings, and 15 squares to order.
How many HardiePlank planks are in a square?
It depends on exposure. With 12 ft planks, James Hardie's coverage chart gives 14 planks per square for 8 1/4 in plank at 7 in exposure (100 ÷ 7 = 14.3), 17 for 7 1/4 in at 6 in (16.7), and 25 for 5 1/4 in at 4 in. The chart excludes waste.
How many HZ5 planks at 7.25 in do I need per square?
A 7 1/4 in HardiePlank HZ5 lap board installs at a 6 in exposure, so each 12 ft plank covers 6 sq ft and a square takes 16.67 planks (the chart rounds to 17). Ten squares take 167 planks before waste.
How many sq ft does one 8.25 in Hardie plank cover?
At the chart's 7 in exposure, a 12 ft plank covers 7/12 ft × 12 ft = 7 sq ft. The other 1 1/4 in is the overlap James Hardie requires as a minimum.
What is a square of siding?
100 square feet of wall. The Vinyl Siding Institute's manual defines a square for ordering, and Hardie's chart counts planks per square too. Divide square feet by 100.
How much does it cost to side a house?
The 2025 Cost vs Value Report priced replacing 1,250 sq ft of siding. James Hardie, citing it, says fiber cement added $24,420 of value at a 113.7% return, which implies a job cost of about $21,478 ($1,718 per square). Consumer Reports, citing the same report, puts the average vinyl job at nearly $18,000 (about $1,440 per square). Local quotes vary, so enter your own price per square in the calculator. Source: Remodeling 2025 Cost vs. Value Report, www.costvsvalue.com.
Should I subtract windows and doors from siding?
The Vinyl Siding Institute says windows and doors usually aren't deducted, because the extra area acts as a waste allowance, but very large openings such as garage doors and sliding doors can be. If you deduct every opening, keep the 10% waste.
How much J-channel do I need?
Add up the perimeter of every window and door: 2 × (width + height). Eight 3 × 4 ft windows and two 3 × 6.75 ft doors need 151 lin ft before waste. The VSI worksheet lists undersill trim separately from J-channel, so check what your product calls for under windows.
How much does lap siding waste?
The VSI manual says to add 10% to all material estimates. James Hardie's plank chart excludes waste entirely. Complex walls with many gables, dormers and openings produce more offcuts than a plain box.
Is this a big-box store siding calculator?
No. It does the same takeoff any store or supplier estimator does, but shows the math and the sources. FreeFast is not affiliated with James Hardie or any retailer; the Hardie plank sizes and coverage come from its published installation instructions.
Sources & method
- James Hardie — HardiePlank HZ5 Lap Siding Installation Instructions (eff. Sept 2019): 12-ft planks, 1 1/4 in min. overlap, coverage chart by width and exposure
- James Hardie — HardiePlank Lap Siding Best Practices Guide: nominal widths 5 1/4–12 in, exposures 4–10 3/4 in
- Vinyl Siding Institute — Vinyl Siding Installation Manual (2016): measuring walls and gables, squares, 10% waste, fasteners (copy hosted by City of Highland Park, MI)
- JLC Cost vs Value 2025 — Siding Replacement | Fiber-Cement (1,250 sq ft project scope)
- JLC Cost vs Value 2025 — Siding Replacement | Vinyl (1,250 sq ft project scope)
- James Hardie — Siding and home value (cites 2025 Cost vs Value: $24,420 value added, 113.7% return)
- Consumer Reports — Siding Buying Guide (cites JLC 2025: vinyl job nearly $18,000; fiber cement tops $21,000)
Results are estimates for general information. Found an error? It helps everyone — see our methodology.