Measure the walls: length and height
You need a 100 ft tape, a notepad and a sketch of the house from above. Siding is ordered by area, so every number you take is either a wall length, a height, or the size of a triangle.
- Sketch the plan. Draw the outline and number each wall. Mark which walls get siding: leave out brick, stone and walls covered by a porch roof.
- Measure each wall along the bottom, corner to corner, at the height where the starter strip will go. Round up to the next half foot. Add them up for the total wall length.
- Measure the wall height from the bottom of the siding to the soffit. Where the ground slopes, measure both ends of that wall and use the average. A two-story wall is one wall; measure it to the upper soffit.
- Count the corners. Outside corners take corner posts. Inside corners, where a wing meets the house, take an inside corner post or J-channel.
Measure gable triangles and dormers by roof pitch
A gable is the triangle of wall above the eave line, at the end of a ridge. Measure its width at the base, which is the width of the wall under it. You do not need to climb to measure its height: find the roof pitch, the inches of rise in 12 in of run, from the plans or with a level and tape against a rafter in the attic. Then:
gable rise = (gable width ÷ 2) × (pitch ÷ 12)
gable area = ½ × gable width × rise
Gables of different widths are separate measurements. A wing is usually narrower than the main house, so its gable is smaller even at the same pitch. The siding calculator takes one gable width, so run the main house first and the odd gables second, then add the areas.
Dormers. A gable dormer has three pieces of siding: the front wall with a small gable over it, and two triangular cheek walls on the sides. Each cheek wall is a right triangle whose height is the dormer's wall height and whose length is that height ÷ (main roof pitch ÷ 12). For example, a 6 ft dormer with 4 ft walls, one window and its own 8/12 roof, set on an 8/12 main roof, has 15 ft² of front after the window, and two cheek walls of ½ × 6 ft × 4 ft = 12 ft² each: 39 ft² in all. Shed dormers have a rectangular front and the same triangular cheeks.
Count doors, windows and other openings
Count every door and window on the walls you are siding, and note the big ones separately. The calculator deducts 21 ft² for a standard door and 15 ft² for a window. A patio door, garage door or picture window is several of those, so measure it and enter it as the number of standard openings with the same area. Leave out small vents and hose bibs; the siding goes around them, and the waste allowance covers the cuts.
Siding take-off for an L-shaped ranch
The example is a one-story L: a 40 × 26 ft main block with its ridge running the long way, and a 16 ft wide wing that projects 14 ft from the front at one end. Walls are 8 ft, the roof is 8/12 throughout, and there are 2 doors and 11 windows. The house has 5 outside corners, so it gets 15% waste. The six walls measure 40 + 26 + 24 + 14 + 16 + 40 = 160 ft.
| Surface | Measurement | Area (ft²) |
|---|---|---|
| Walls | 160 ft × 8 ft | 1,280 |
| Main gables | 2 × ½ × 26 ft × 8' 8" rise | 225 |
| Wing gable | ½ × 16 ft × 5' 4" rise | 43 |
| Doors and windows | 2 × 21 + 11 × 15 | −207 |
| Net siding area | 1,548 − 207 | 1,341 |
| Squares with 15% waste | 13.41 × 1.15 | 15.42 sq |
| Vinyl boxes | 15.42 ÷ 2, rounded up | 8 boxes |
Entering all three gables at the main width would have counted the wing gable as 113 ft² instead of 43, for 1,411 ft² net. Measuring each gable keeps that extra off the order. Add the rounding at the end: round the combined squares up to boxes once, not each run separately.
How trim is counted: starter strip, J-channel and corner posts
Trim is ordered by length, from the same measurements.
| Item | Where it goes and how it is measured | Length | Pieces |
|---|---|---|---|
| Starter strip | Along the bottom of every sided wall: the total wall length | 160 ft | 16 × 10 ft |
| J-channel | Around 2 doors and 11 windows (210 ft) and up the gable rakes (81.7 ft) | 291.7 ft | 24 × 12' 6" |
| Outside corner posts | One per outside corner per 10 ft of wall height | 5 corners | 5 |
| Inside corner | Where the wing meets the main block | 1 corner | 1 |
The rake length is the sloped edge of each gable: half the gable width × the slope factor, twice per gable. For the 26 ft gables that is 2 × 13 × 1.202 = 31.2 ft each. Add the J-channel total before dividing by the 12' 6" piece length, as here, and you buy fewer pieces than if you round each opening up. Finish trim under the soffits, and any soffit and fascia, are separate items measured along the eaves.
If your rakes are long or steep, the roof area calculator lists the slope factor for each pitch.
Sources
- Geometry: wall area = wall length × height; gable rise = (width ÷ 2) × (pitch ÷ 12); gable area = ½ × width × rise; slope factor = √(1 + (pitch ÷ 12)²)
- Siding calculator constants: 21 ft² per door and 15 ft² per window; J-channel 17 ft per door and 16 ft per window; starter strip 10 ft; J-channel 12' 6"; corner posts 10 ft; 2 squares per box
- Roof pitch slope factors