How siding is calculated
Siding is ordered by area. Find the area of the rectangular walls, add the triangles at the gable ends, take out the doors and windows, then add waste. The trim pieces are counted from lengths: the perimeter for starter strip, the openings and rakes for J-channel, and the corners for corner posts.
If parts of the house have different wall heights, such as a two-story wing or an attached garage, enter each part with “+ Add another wall section”. Each section has its own walls, gables, openings and corners; the areas and lengths are added together before boxes, planks and trim are rounded up once for the whole job.
wall area = total wall length × wall height
gable rise = (gable width ÷ 2) × (pitch ÷ 12)
gable area = gable ends × ½ × gable width × rise
net area = wall area + gable area − doors × 21 ft² − windows × 15 ft²
squares = net area ÷ 100 with waste = squares × (1 + waste % ÷ 100)
Vinyl is sold by the box and lap siding by the plank, so the last step depends on the product. Both round up to whole pieces.
vinyl boxes = squares with waste ÷ 2, rounded up
lap planks = net area with waste ÷ (exposure in inches ÷ 12 × plank length), rounded up
Trim is counted the same way for both products:
starter strip = total wall length ÷ 10 ft, rounded up
slope factor = √(1 + (pitch ÷ 12)²)
J-channel = doors × 17 ft + windows × 16 ft + gable ends × gable width × slope factor
J-channel pieces = J-channel ÷ 12.5 ft, rounded up
corner posts = outside corners × (wall height ÷ 10 ft, rounded up)
The door allowance of 17 ft is two 7 ft sides plus a 3 ft head. A 3 × 5 ft window has 16 ft of perimeter. The gable term is the two sloped rakes of each gable.
Worked example: house with 140 ft of wall
The calculator opens on a house with 140 ft of wall, 9' walls, 2 gable ends 30' wide at 6/12, 2 doors, 10 windows and 10% waste.
- Walls: 140 × 9 = 1,260 ft².
- Gable rise: 15 × (6 ÷ 12) = 7.5 ft. Gables: 2 × ½ × 30 × 7.5 = 225 ft².
- Openings: 2 × 21 + 10 × 15 = 192 ft². Net area: 1,485 − 192 = 1,293 ft².
- Squares: 12.93, or 14.22 with 10% waste. Vinyl boxes: 14.22 ÷ 2 = 7.11, so order 8 boxes.
- Starter strip: 140 ÷ 10 = 14 pieces.
- J-channel: 2 × 17 + 10 × 16 + 2 × 30 × 1.118 = 261.1 ft, or 21 pieces at 12' 6".
- Corner posts: 4 corners × 1 = 4 posts.
Switch the siding type to lap planks and the same house needs 1,422.3 ft² ÷ 7 ft² per plank = 204 planks of 12 ft at 7 in exposure. The trim counts do not change.
Siding for common house sizes
| Footprint (ft) | Walls + gables (ft²) | Squares with waste | Vinyl boxes |
|---|---|---|---|
| 24 × 36 | 1,224 | 13.46 | 7 |
| 28 × 44 | 1,492 | 16.41 | 9 |
| 30 × 50 | 1,665 | 18.32 | 10 |
| 32 × 60 | 1,912 | 21.03 | 11 |
Measuring tips
- Measure each wall along the bottom, at the height where the starter strip goes, and add them up for the total wall length. Skip walls that get brick, stone or no siding. Walls of a different height go in their own wall section.
- Wall height runs from the bottom of the siding to the soffit. If the grade slopes, average the high and low ends of that wall.
- A gable end is the triangle above the eave line. Its width is the wall width under it. Read the pitch from the plans or with a level and tape on the rake, or check the pitch table on the roof area calculator.
- Count every door and window, but measure the big ones. A 16 × 7 ft garage door is about 5 standard doors of area.
- Count outside corners only. Inside corners are usually finished with J-channel or an inside corner post, so add them to your J-channel or buy them separately.
Waste and ordering
Waste covers the offcuts at every opening, the angled cuts on the gables and the overlaps where panels lap at the ends. Start at 10% on a plain rectangle with two gables and four corners. Go to 15% for a house with extra gables, bump-outs, bay windows or more than four outside corners; the calculator flags low waste on those houses.
Order full boxes and full lengths of trim, and buy from the same production run so the colour matches. Check whether the supplier takes back unopened boxes. Follow the instructions printed with your siding for nail spacing, expansion gaps and clearance above grade.
Sources
- Geometry: wall area = perimeter × height; gable triangle area = ½ × width × rise, rise = (width ÷ 2) × (pitch ÷ 12)
- Industry norms: deduct 21 ft² per standard door (3 × 7 ft) and 15 ft² per window (3 × 5 ft); waste 10% for simple houses, 15% with many corners or gables
- Vinyl siding packaging and accessories: 1 box ≈ 2 squares (200 ft²); starter strip 10 ft; J-channel 12' 6"; outside corner posts 10 ft
- Lap planks: coverage per plank = exposure × length (8¼ in plank at 7 in exposure, 12 ft long)