Siding Square Footage Calculator
The net area to side, in square feet: rectangular walls plus gable triangles, minus the door, window and garage openings. This is the base of every quantity and cost estimate.
Your siding square footage is about 1,336 sq ft — add the walls (perimeter × height) and the gable triangles, then subtract the doors, windows and garage openings. This net area is the base of every quantity and cost estimate that follows.
1 Enter your numbers
Square footage is the foundation number for a siding job. Every other figure — squares, boxes, bundles, house wrap, and the whole cost estimate — is this net area multiplied or divided by something. Get it right and the rest is arithmetic; get it wrong and the error propagates into every quote you compare.
The method is deliberately simple plane geometry, because siding walls are simple shapes. Rectangles for the walls, triangles for the gables, and a subtraction for the openings. This tool keeps the three parts visible — wall area, gable area, deductions — so you can audit each one instead of trusting a single black-box total.
Gross vs net
Gross area is all the wall surface: walls plus gables. Net area is what you actually cover with siding: gross minus the openings. Contractors quote and order against the net area (plus waste), so that is the number to carry forward. If you want a deliberately conservative order, side the small windows — leave them in the area — and treat the extra as waste margin.
Formula
wall_area = perimeter × wall_height
gable_area = Σ (½ × base × height)
net_area = wall_area + gable_area − Σ openings
Openings use labeled standard deductions (door ≈ 21, window ≈ 15, double garage ≈ 112 sq ft); swap in your measured sizes for accuracy. Net area is floored at zero — a wall cannot have negative siding.
Worked example
For a 140 ft perimeter, 10 ft walls, two 30 × 8 ft gables, 10 windows, 2 doors and 1 double garage door:
- Walls: 140 × 10 = 1,400 sq ft
- Gables: 2 × ½ × 30 × 8 = 240 sq ft
- Openings: (10 × 15) + (2 × 21) + 112 = 304 sq ft
- Net: 1,400 + 240 − 304 = 1,336 sq ft
That 1,336 sq ft is the net area to side — feed it into the squares, boxes and cost tools.
What to measure, and where it goes wrong
- Perimeter, not floor plan. Measure the walls as built; bump-outs, offsets and bay windows add wall the plan understates.
- One height, honestly. Use the wall height to the soffit. On a sloped grade, measure the taller side or average carefully — the shortest measurement is safest for ordering.
- Gables are extra, not included. The perimeter × height rectangle stops at the eaves; the triangle above it is separate.
- Deduct real openings. A wall of glass or a wide slider removes far more than a standard window — measure large openings individually.
Reference table
Labeled planning typicals — measure your actual openings. Subtract these from the gross wall + gable area.
| Opening | Deduct (sq ft) |
|---|---|
| Standard door (3 × 7) | 21 |
| Standard window | 15 |
| Patio / sliding-glass door | 40 |
| Single garage door (9 × 7) | 63 |
| Double garage door (16 × 7) | 112 |