Siding is wall area minus openings, divided into "squares" of 100 square feet — but the parts that trip people up are gable triangles, the trim that wraps every edge, and waste factors that swing widely between lap, vinyl, and board-and-batten. This guide covers area, squares, trim, and cost.
Start with the rectangular wall faces: building perimeter times wall height. Then add the triangular gable ends, which beginners routinely forget. A gable is a triangle, so its area is half the base times the height to the peak.
Walls: perimeter (30+40)×2 = 140 ft × 9 ft = 1,260 sq ft. Gables: 2 × (½ × 30 × 6) = 2 × 90 = 180 sq ft. Total ≈ 1,440 sq ft before openings — about 14.4 squares.
Like roofing, siding is counted in squares, where one square covers 100 square feet of wall. Vinyl siding ships in boxes that cover about 2 squares (200 sq ft); fiber-cement and wood lap come in bundles by linear foot. Convert your total area to squares, add waste, and round up to whole boxes or bundles.
Subtract only the large openings — picture windows, sliding doors, and garage doors. Standard windows and entry doors are usually left in the gross area because siding is cut around them and the offcuts are scrap; the waste factor covers it. If you have many large openings, deducting them avoids badly over-ordering.
| Siding type / condition | Waste |
|---|---|
| Vinyl or aluminum lap, simple walls | 10% |
| Fiber-cement or wood lap | 10–12% |
| Many dormers, corners, cut-up walls | 15% |
| Diagonal or board-and-batten | 15%+ |
Lap siding's coverage depends on the exposure (the visible height of each course). A board with a 7-in exposure covers less wall than the same board at 8 in. Confirm the planned exposure before converting linear feet to area.
Trim is where siding estimates get detailed. Measure these by the linear foot off the elevations:
Behind the siding goes a weather-resistive barrier (house wrap), sold in rolls commonly 9 ft × 100 ft (≈900 sq ft) or 3 ft × 100 ft. Divide wall area by roll coverage and round up, allowing overlap at seams. Fasteners are siding nails or screws sized to the material; vinyl needs roofing nails with a wide head left slightly loose for expansion.
| Item | Typical figure |
|---|---|
| Vinyl siding (material) | $1.50–$4 / sq ft |
| Fiber-cement (e.g. lap) | $3–$7 / sq ft |
| Wood / engineered wood | $4–$9 / sq ft |
| House wrap | $0.15–$0.40 / sq ft |
| Install labor | $2–$5 / sq ft |
Use the siding calculator to turn wall and gable measurements into squares and a regional cost, then itemize the trim by linear foot from your elevations.