How to measure a house for siding
Walk round the outside with a long tape and write down each wall. Split anything that is not a plain box into rectangles, so a bump-out or an attached garage is its own set of walls. Kaycan's guide rounds every measurement up to the next 3 inches. Do the same and small errors will not leave you short.
Height runs from the bottom edge of the siding to the eaves, not from the ground. For a gable, you need the width at the widest point, including the roof overhang, and how steep the roof is. If you cannot reach a gable to measure it, a roofer or an old inspection report can usually give you the pitch.
The question people most often ask is how much siding a house of so many square feet takes. Floor area cannot answer it. A two-storey house has half the footprint of a single-storey one of the same floor area, and so a very different perimeter and wall height.
A worked example
This is the example house in Kaycan's estimating guide: 40 by 30 feet, 9 feet to the eaves, with a 1 foot overhang and a 5 in 12 roof. These lines come from the same formula the calculator above runs.
Kaycan's 40 by 30 ft house
- Walls: 2 × (40 + 30) × 9 = 1260 sq ft
- One gable: 32 × 6.67 × 0.75 = 160 sq ft
- Two gables: 320 sq ft
- Total: 1580 sq ft, so 16 squares
Kaycan rounds the slope to 0.42 and reaches 1,582.4 square feet and the same 16 squares. As the guide says, because nothing was deducted for openings and everything was rounded up, there is no need to add more for cutting waste. If you are also re-covering the roof, the roofing calculator uses the same measurements, and it often makes sense to look at wall insulation with the insulation calculator while the walls are open.
Siding for common house sizes
These all use two gables at 5 in 12 with a 1 foot overhang, and no openings taken off. Each two-storey house has the footprint of the ranch above it, so the gables match and only the walls change.
| House | Walls | Gables | Squares | 7 in planks |
|---|---|---|---|---|
| Shed, 8 × 10 | 288 | 31 | 4 | 46 |
| Garage, 24 × 24 | 864 | 211 | 11 | 154 |
| Ranch, 1,000 sq ft | 1170 | 228 | 14 | 200 |
| Ranch, 1,500 sq ft | 1440 | 320 | 18 | 252 |
| Two storeys, 2,000 sq ft | 2340 | 228 | 26 | 367 |
| Two storeys, 3,000 sq ft | 2880 | 320 | 32 | 458 |
Mistakes that throw off an order
- Starting from floor area. Wall area comes from the perimeter and the height. Skip the shortcut.
- Deducting every window and adding no waste. That is more aggressive than any maker's method we read. The usual approach leaves them in as your allowance.
- Treating a gable as a triangle. Half the rectangle is the true area, but cutting along the rake wastes much of each panel, and low pitches waste more.
- Forgetting trim. Starter strip runs the whole perimeter, J-channel goes round openings and gable ends, and soffit is a separate order altogether.
- Reading the piece count as exact. It holds for 12 foot planks at one exposure. Panel sizes vary by profile and maker, so check yours.
Questions
What is a square of siding?
A square is 100 square feet of siding. Siding is ordered in squares in the same way roofing is, so a house needing 1,580 square feet needs 15.8 squares, which you round up to 16.
How much siding do I need for a 1,500 square foot house?
About 18 squares for a single-storey 50 by 30 foot house with 9 foot walls and two gables at 5 in 12. Floor area alone cannot give the answer, because siding depends on the perimeter and wall height, so a two-storey house of the same floor area needs a different amount.
Do I subtract windows and doors?
Usually not. The PEPA manual says windows and doors are not usually deducted, because including them gives an allowance for waste. Take off something very large, such as a garage door. Fiber cement charts such as James Hardie's are stated less openings, so ask if you are ordering that.
How do I measure a gable?
Multiply the gable's width by its height. For ordering, Kaycan then multiplies by 0.75, where a true triangle would be half, to allow for waste cutting along the sloped edge. The calculator works out the height from the width and the roof pitch.
How many Hardie planks are in a square?
At 7 inch exposure, 14 planks of 12 feet make a square, by James Hardie's coverage chart, and at 5 inch it is 20. The sum is 100 divided by the exposure in inches. The chart excludes waste, so add your own allowance for that.
How much waste should I add for siding?
Add nothing if you measured as this calculator does and took off no openings, by Kaycan's guide. CertainTeed suggests you may want to add 10 percent to siding and accessories. Lower-pitch roofs make more waste than steep ones, and complicated houses more than plain ones.
This siding order is a planning estimate for a simple rectangular house with gables. Dormers, bump-outs and low-pitch gables change it, and panel coverage differs by profile and maker. Have your supplier take off the real house from a drawing before you order, and read the coverage on the product's own label.
Sources and review
- Vinyl siding installation manual, Polymeric Exterior Products Association, 2022.
- Vinyl siding estimating and installation guide, Kaycan, 2025.
- Estimating and installing vinyl siding, CertainTeed, no date shown, read November 2026.
- HardiePlank lap siding product description, James Hardie, no date shown, read November 2026.
Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.
How this is calculated
Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.
The arithmetic
Walls
Walls = 2 × (Length + Width) × Height
- Height
- Wall height to the eaves, in feet
Worked through: 2 × (40 + 30) × 9 = 1260 sq ft
One gable, with cutting allowance
Gable = Width × Rise × 0.75
- Width
- House width plus the overhang on both sides
- 0.75
- Kaycan's allowance for waste along the rake
Worked through: 32 × 6.67 × 0.75 = 160 sq ft
Squares
Squares = Area ÷ 100, rounded up
- 100
- Square feet in a square
Worked through: 1580 ÷ 100 = 15.8, so 16 squares
The constants, and where they come from
SQ_FT_PER_SQUARE
Published figureSquare feet in one square of siding
Applies to: The way siding is ordered in the United States
Source: Vinyl siding installation manual, Polymeric Exterior Products Association, published 2022, read 2026-11. This source is about 4 years old, and it is the one the figure rests on.
The trade's definition, printed in the PEPA manual. PEPA was formerly the Vinyl Siding Institute, and a new manual was announced for fall 2026, which we did not read.
GABLE_ALLOWANCE
Published figureShare of the gable's bounding rectangle to order, allowing for cuts along the rake
Applies to: Horizontal siding on a triangular gable
Source: Vinyl siding estimating and installation guide, Kaycan, published 2025, read 2026-11.
Also from: Estimating and installing vinyl siding, CertainTeed, no date on the page, read 2026-11.
A true triangle is 0.5. Kaycan prints 0.75 to allow for waste. CertainTeed gets to a similar place by adding a foot of height, and says lower pitches waste more.
PLANK_FEET
Published figurePlank length used to count pieces, in feet
Applies to: Fiber cement lap siding
Source: HardiePlank lap siding product description, James Hardie, no date on the page, read 2026-11.
James Hardie's coverage chart counts 12 foot planks. Other products differ, and the piece count is for this length only.
INCHES_PER_FOOT
DefinitionalInches in one foot
Applies to: Definitional, everywhere
True by definition, so there is nothing to cite. The derivation is below.
Twelve inches make a foot by definition, and a roof pitch is a rise over 12 inches of run.
What the labels mean
- Published figure:
- taken directly from the source named beside it.
- Definitional:
- true by definition, like the number of inches in a foot, so there is nothing to cite.
If a figure here looks wrong, it may be. Tell us what you found and we will check it against the source and publish a correction if it needs one. How constants are chosen and how often they are reviewed is set out in our methodology.
How to use the siding calculator
- 1
Walk round the house with a tape and note its length and width
- 2
Measure the height from the bottom of the siding to the eaves
- 3
Count the gable ends and find the roof pitch
- 4
Read the squares to order and the plank count
- 5
Add any large opening or extra waste in the further section
What this calculator assumes
Every estimate here rests on the assumptions below. Where your job differs from one of them, the figure will differ too.
- The house is a rectangle with straight walls, and every wall gets siding
- Doors and windows are not taken off, as the PEPA manual and Kaycan advise. Their area acts as the allowance for waste
- A gable counts as 0.75 of its bounding rectangle, which allows for cuts along the rake
- Plank count assumes 12 foot planks at the exposure you choose. Other lengths change the count
Pro tips
- Round each measurement up, as Kaycan does
- Measure from the bottom edge of the siding, not the ground
- Keep a few extra planks for later repairs, since profiles and colours get discontinued