Siding Calculator
Estimate the siding for a house, garage or shed. List the walls and gables, subtract doors, windows and the garage door, and get the net area, the number of siding squares and panels, plus J-channel, starter strip and corner posts.
Triangles above the walls: base = wall width, height = peak above the wall top. Leave empty if none.
A 16 × 7 ft garage door is 112 sq ft.
Exposure (ft) × panel length (ft).
10% for simple walls, 15% with many gables and openings.
How to use the Siding Calculator
- List each wall as width × height in feet, one per line.
- Add gables as base × height of the triangle above the wall (leave empty if none).
- Enter doors and windows with their typical size, and any garage door area.
- Choose the siding type and a waste allowance, and optionally a price.
- Read squares, panels and trim, and check each surface in the table.
Siding formulas
- Wall area = width × height for each wall
- Gable area = ½ × base × height for each triangle
- Net area = walls + gables − doors − windows − other openings
- Squares = net area × (1 + waste) ÷ 100
- Panels = net area × (1 + waste) ÷ coverage per panel, where coverage = exposure × length
Example: a 40 × 28 ft house with 9 ft walls: 2 × 40 × 9 + 2 × 28 × 9 = 1,224 sq ft, plus two gables of ½ × 28 × 7 = 196 sq ft, minus 2 doors (42 sq ft) and 10 windows of 3 × 4 ft (120 sq ft) = 1,258 sq ft. With 10% waste that is 1,384 sq ft = 13.84 squares, or 167 double 4 in vinyl panels.
Siding panel coverage
| Siding | Exposure | Length | Coverage per piece | Pieces per square |
|---|---|---|---|---|
| Vinyl double 4 in | 8 in | 12 ft 6 in | 8.33 sq ft | 12 |
| Vinyl double 5 in | 10 in | 12 ft | 10 sq ft | 10 |
| Fiber cement lap 8.25 in | 7 in | 12 ft | 7 sq ft | 14.3 |
| Fiber cement lap 6.25 in | 5 in | 12 ft | 5 sq ft | 20 |
| Fiber cement lap 7.25 in | 6 in | 12 ft | 6 sq ft | 16.7 |
Panel lengths and exposures vary by manufacturer, so check the product label and use the custom coverage option if yours differs. Trim items such as soffit, fascia and window trim are measured separately.
Frequently asked questions
How do I calculate how much siding I need?
Multiply the width and height of each wall, add the gable triangles (½ × base × height), subtract doors and windows, then add 10-15% for waste. Divide by 100 to get squares, the unit siding is sold in.
What is a square of siding?
A square is the amount of siding that covers 100 square feet of wall. Vinyl siding is usually sold in boxes of 2 squares (200 sq ft).
How do I calculate the area of a gable?
A gable is a triangle: area = ½ × base × height. A gable on a 28 ft wide wall that rises 7 ft at the peak is ½ × 28 × 7 = 98 sq ft.
Should I subtract windows and doors from siding?
Yes, for an accurate estimate, especially with large openings like garage doors and patio doors. Some installers do not subtract small windows because the cut pieces are often waste anyway; a higher waste allowance has the same effect.
How much waste should I add for siding?
Add about 10% for simple rectangular houses and 15% or more for houses with many gables, dormers, corners or openings, because each cut leaves an offcut.
How many vinyl siding panels are in a square?
With double 4 in panels 12.5 ft long, each panel covers about 8.33 sq ft, so a square takes 12 panels. Double 5 in panels 12 ft long cover 10 sq ft each, 10 per square.
What is J-channel used for?
J-channel is trim that holds the cut ends of siding around windows, doors and along roof lines. Measure the sides and top of each door and all four sides of each window to estimate it.