How to use the siding calculator
Enter the house's length, width and wall height to the eaves, then how many gable ends will be sided and the roof pitch. The calculator adds the rectangular walls and the triangular gables, subtracts the openings you enter, adds waste and converts to squares. It also estimates the trim: corner posts for the four outside corners, starter strip along the bottom, and J-channel around windows and doors.
For an L-shaped house or one with additions, run each rectangular block separately. Each extra outside corner adds a corner post.
The formulas
walls = 2 × (length + width) × wall height
gable = ½ × width × (width ÷ 2 × pitch ÷ 12)
squares = (walls + gables − openings) × (1 + waste) ÷ 100
Worked example: a 40 × 30 ft one-story house with two gables at 6/12
- Walls: 2 × (40 + 30) × 9 = 1,260 ft².
- Each gable: 30 ft wide, 15 × 6/12 = 7.5 ft high, so ½ × 30 × 7.5 = 112.5 ft²; two gables 225 ft².
- Less 60 ft² of openings: 1,425 ft².
- With 10% waste: 1,567.5 ft², or 15.7 squares.
Trim and accessories
| Item | Where | How it's counted |
|---|---|---|
| Starter strip | Bottom of every wall | House perimeter |
| Outside corner posts | Each outside corner | Wall height, usually 10 ft pieces |
| J-channel | Around windows and doors, and along gable rakes | Perimeter of each opening |
| Soffit and fascia | Under the eaves | Eave length × overhang |
| House wrap | Over the sheathing | Same area as the siding, plus laps |
J-channel here is estimated at 16 ft per window or door (a typical 3 × 5 ft window has a 16 ft perimeter) plus the gable rakes. Large patio doors need more.
Measuring the house
- Measure each wall's length at the foundation and its height from the bottom of the siding to the soffit.
- For gables, measure the width at the base and either the pitch or the height from the top of the wall to the peak.
- List the large openings (garage door, sliding doors) with their sizes; count the regular windows and doors for J-channel.
- Note every outside and inside corner, since each needs its own trim piece.
Tips
- Measure each wall rather than assuming the house is symmetrical; additions and bump-outs add up.
- Vinyl expands. Leave gaps at channels and don't nail tight, per the manufacturer's instructions.
- Buy one lot. Order all the siding at once so the color matches.
Questions people ask
How do I calculate siding for a house?
Add up the wall areas (perimeter × wall height), add the triangular gable ends (½ × width × gable height), subtract doors and windows, and add about 10% for waste. Divide by 100 to get squares, the unit siding is sold in.
What is a square of siding?
100 square feet of wall coverage. A 1,500 square foot wall area needs 15 squares before waste.
How much waste should I add for siding?
About 10% for a simple rectangular house, more for houses with many corners, gables, dormers and openings. Gable ends in particular produce lots of angled offcuts.
How do I calculate the area of a gable?
A gable end is a triangle: ½ × the wall width × the height from the top of the wall to the peak. For a 30 ft wide gable at 6/12 pitch, the height is 7.5 ft and the area 112.5 square feet.
Should I subtract windows and doors?
Subtract large openings such as garage doors and patio doors. For average windows many installers don't subtract, since the cut-out pieces are waste and trimming around them uses extra material.
Sources and assumptions
- A square is 100 ft² of coverage. Carton coverage varies by profile and maker; enter the squares per carton from your product to get a carton count.
- Gable height = half the gable width × pitch ÷ 12 (right-triangle geometry).
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.