Siding Calculator
Work out exactly how many siding panels or boards, and how much starter strip and J-channel, you need for any house, with the standard waste allowance already built in. Add your own prices for a full cost estimate — nothing here uses a stale national average.
How this calculator works
The calculator sums the area of every wall, subtracts openings, then divides by the coverage of a single panel or board and adds 10% for waste. Starter strip and J-channel are worked out separately, from wall widths and opening perimeters:
area = sum(walls) − openings panels = ceil( area / coverage_per_panel × 1.10 ) // 10% waste starter_strip_ft = perimeter (sum of wall widths at base) j_channel_ft = perimeter of all openings
Starter strip only needs the width of each wall at its base, not its height, since it's a single strip that runs along the bottom of every wall run. J-channel is sized from each opening's own perimeter, since it frames every window and door individually.
Worked example
A house with front and back walls 30 ft × 9 ft, side walls 20 ft × 9 ft, three 3×4 ft windows, and one 3×7 ft door, in vinyl panel:
- Wall area: 2 × (30 × 9) + 2 × (20 × 9) = 900 sq ft.
- Openings: 3 windows × 12 sq ft + 1 door × 21 sq ft = 57 sq ft subtracted.
- Net siding area: 900 − 57 = 843 sq ft.
- At 16 sq ft per panel with 10% waste: ceil(843 ÷ 16 × 1.10) ≈ 58 panels.
- Starter strip: 30 + 30 + 20 + 20 = 100 ft. J-channel: 3 × 2(3+4) + 2(3+7) = 62 ft.
At $9 per panel and $1.10 per linear ft of trim, that example totals 58 × $9 + 162 × $1.10 ≈ $700.
Vinyl vs lap board coverage
| Type | Typical coverage | Why |
|---|---|---|
| Vinyl panel | ~16 sq ft | Fixed nominal panel size across most profiles |
| Lap board (12 ft, 7 in reveal) | ~7 sq ft | Coverage is board length × exposed reveal |
Because lap boards cover so much less area per piece, a lap-sided house needs several times more pieces than the same house in vinyl — the piece count isn't a sign of a bigger job, just a smaller unit of coverage.
Measuring your house
The same formula works for any house shape — only the number of walls and openings changes:
- Walls — measure each side of the house as its own entry, width by height to the eave line.
- Gables — add a triangular gable as its own wall using its average height, or measure it separately if your layout tool supports it.
- Openings — measure the rough opening for every window and door; don't forget smaller openings like vents.
- Starter strip — only needed once per wall run, along the base — it doesn't scale with height.
How much does new siding cost?
Rather than quoting a national average that's stale the day it's published, this calculator prices the job from what you actually enter. Give a price per panel or board and a single trim price per linear foot, which covers starter strip and J-channel together — each is itemized and totaled. The estimate doesn't include house wrap, flashing, fasteners, or removal of existing siding.
Frequently asked questions
Vinyl vs fiber cement — does coverage per piece differ a lot?
Yes. A vinyl panel typically covers around 16 square feet. Fiber cement or wood lap boards cover much less per piece — often just 6 to 8 square feet for a 12 ft board with a 6–8 inch reveal — so lap siding needs more pieces per square foot than vinyl.
How much waste should I plan for around windows and corners?
A standard 10% allowance, built into this calculator, covers typical cuts around openings and at outside corners. Homes with lots of windows, gables or other cut-heavy details often push closer to 12–15%.
What is J-channel and how much do I need?
J-channel is the trim that frames window and door openings and other panel terminations, giving siding a finished edge to lock into. The footage needed equals the perimeter of every opening added together, calculated automatically here.
Do I need house wrap included in this estimate?
No — this covers siding, starter strip and J-channel only. Prep materials like house wrap, flashing tape and fasteners aren't included, since they're typically priced separately by the roll or box.