Siding Calculator
Siding = walls + gables − half the large openings, + 10 % waste. 140 ft of walls 9 ft high + 2 gables 30 ft wide, 7.5 ft high = 1,485 ft² → 17 squares. Leave windows and ordinary doors in the area, as the waste allowance: only garage and sliding doors come off, at half.
Vinyl siding to order
17 squares
1 square = 100 ft² (9.29 m²) of wall
- Walls
- 1,260 ft²
- Gables
- 225 ft²
- Net wall area
- 1,485 ft²
- To order, with 10 % waste
- 1,633.5 ft²
Starter strip, J-channel and corner posts are sold by the linear foot: measure them separately.
Worked examples
A 40 × 30 ft house, walls 9 ft high, two gables
- Walls: 140 × 9 = 1,260 ft²
- Gables: 2 × ½ × 30 × 7.5 = 225 ft²
- 1,260 + 225 = 1,485 ft²; + 10 % = 1,633.5 ft²
- 1,633.5 ÷ 100 = 16.3, so 17 squares
Result: 17 squares of vinyl siding.
Checked by the automated test “siding — defaults: 140 ft of walls 9 ft high + 2 gables 30 × 7.5 ft, 10 % waste → 1,485 ft², 17 squares of vinyl”.
The same house in 8 1/4 in HardiePlank
- Each 12 ft plank covers 7 in × 12 ft = 7 ft²
- 1,633.5 ÷ 7 = 233.4, so 234 planks
Result: 234 planks of 12 ft.
Checked by the automated test “siding — fiber cement 8 1/4 in planks (7 in exposure), same house → 234 planks of 12 ft”.
How it works
Walls = length of the walls × height
Gable = ½ × width × height, for each gable
Net area = walls + gables − ½ × garage and sliding doors
Vinyl squares = net area × (1 + waste) ÷ 100 ft², rounded up
Lap planks = net area × (1 + waste) ÷ (exposure × 12 ft), rounded up
Assumptions
- Estimating worksheet of the vinyl siding industry (PEPA, formerly the Vinyl Siding Institute): windows and ordinary doors are not deducted, garage and sliding doors count at half their area; 10 % waste is the usual allowance.
- Fiber cement planks are 12 ft long; the exposure (the part left visible) is the plank width minus the 1 1/4 in lap of the James Hardie chart.
- Trim, starter strip, J-channel and corners are not in the count: they are sold by the linear foot.
Reference table
| Plank | Exposure | Planks per 100 ft² | With 10 % waste |
|---|---|---|---|
| 5 1/4 in | 4 in | 25 | 28 |
| 6 1/4 in | 5 in | 20 | 22 |
| 7 1/4 in | 6 in | 16.7 | 19 |
| 7 1/2 in | 6 1/4 in | 16 | 18 |
| 8 in | 6 3/4 in | 14.8 | 17 |
| 8 1/4 in | 7 in | 14.3 | 16 |
| 9 1/4 in | 8 in | 12.5 | 14 |
| 12 in | 10 3/4 in | 9.3 | 11 |
Without waste, these are the James Hardie chart figures for one square (it rounds 16.7 to 17 and 14.3 to 14).
Common mistakes
- Deducting every window and door The PEPA worksheet keeps windows and doors in the wall area: the trim around them uses that siding. Only garage and sliding doors come off, at half: a 16 × 7 ft garage door is 112 ft², 56 ft² deducted.
- Forgetting the gables Two gables 30 ft wide and 7.5 ft high add 225 ft² of wall. Without them, the 40 × 30 ft house above orders 14 squares instead of 17.
- Taking the plank width for the exposure An 8 1/4 in HardiePlank shows 7 in: the James Hardie chart counts a 1 1/4 in lap. Divided by 8 1/4 in, the same house gets 198 planks instead of 234.
- Counting trim in squares Starter strip, J-channel and corner posts are sold by length. PEPA measures the starter strip around the whole base of the house: 140 ft for a 40 × 30 ft house.
Frequently asked questions
How much siding do I need?
Add the walls (length × height) and the gables (½ × width × height each), take off half of any garage or sliding door, then add 10 %. A 40 × 30 ft house with 9 ft walls and two gables: 1,485 ft², 1,633.5 ft² to order.
How many squares of siding do I need?
One square covers 100 ft². Divide the wall area plus 10 % waste by 100 and round up: 1,633.5 ft² is 17 squares, and 2,000 ft² of wall takes 22.
How much Hardie siding do I need?
Divide the wall area plus 10 % by what one 12 ft plank covers, exposure × 12 ft. An 8 1/4 in plank shows 7 in, so 7 ft²: 1,633.5 ft² takes 234 planks. James Hardie's chart gives 14 planks per square, before waste.
Sources
Formula, examples and sources checked on by the Yardstick Math team. How we check our formulas.
- Vinyl Siding Installation Manual 2022 (estimating worksheet: windows and doors not deducted, garage and sliding doors × 0.50, 10 % waste, squares of 100 ft²) — Polymeric Exterior Products Association (PEPA), accessed
- HardiePlank lap siding installation, coverage chart (number of 12 ft planks per square) — James Hardie, accessed