Siding Estimate: Wall Area, Gables, Openings, Waste, and Cost
October 29, 2026
Estimating siding starts with the total wall area, from which you subtract the openings and add a waste allowance. Siding is sold by the square (100 square feet) or by the piece, so convert your area accordingly.
For a rectangular house of 40 by 30 feet with 9-foot walls: perimeter 2 × (40 + 30) = 140 feet, wall area 140 × 9 = 1,260 square feet. Add the two triangular gable ends: each 30 feet wide and 8 feet tall, ½ × 30 × 8 = 120, so 240 for both.
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Gross area: 1,260 + 240 = 1,500 sq ft. Openings: 10 windows at 15 sq ft = 150; 2 entry doors at 21 sq ft = 42; one garage door of 7 × 16 = 112; total 304 sq ft. Net area: 1,500 − 304 = 1,196 sq ft. Waste: add 10% for straight runs (up to 15% for complex layouts with many corners, or diagonal patterns): 1,196 × 1.10 = 1,316 sq ft, or 13.16 squares; order 14 squares. Some estimators don't subtract small openings (less than 10 sq ft) because the cutting waste is similar.
Add trim: corners (each outside corner needs a corner post, about 4 for a rectangle), J-channel around windows and doors, starter strips along the bottom (140 feet), and soffit and fascia if replacing. Measure the linear feet.
| Step | Calculation | Result |
|---|---|---|
| Wall area | 140 ft x 9 ft | 1,260 sq ft |
| Gables | 2 x (1/2 x 30 x 8) | 240 sq ft |
| Openings | 150 + 42 + 112 | 304 sq ft |
| Net area | 1,500 - 304 | 1,196 sq ft |
| With 10% waste | 1,196 x 1.10 | 1,316 sq ft = 13.2 squares |
Cost Estimate
Installed siding costs vary widely: vinyl $3 to $8 per square foot, fiber cement $6 to $13, wood $7 to $15, and engineered wood somewhere in between. At $5 per sq ft installed, 1,316 sq ft costs $6,580. Add removal of old siding, house wrap, flashing, and repairs for rot. Get three quotes and check what is included. Material-only costs for vinyl might be $1.50 to $3 a square foot, so doing it yourself saves labor but needs skill.
Siding Pieces
Lap siding is sold in panels that cover a specified area; vinyl panels, for example, are sold in cartons that cover a stated area. Check the label for the actual coverage per carton instead of assuming. Divide the net area by the coverage and round up.
Tips
Measure each wall separately for houses with bumps and bays, and sketch the elevations. Order all siding at once from the same batch to match colors. Keep leftover pieces for repairs. Check local codes for permits and for required housewrap and flashing details.
Frequently Asked Questions
How do I calculate the siding area?
Wall perimeter times height plus gables, minus openings, then add waste.
What is a square of siding?
100 square feet.
How much waste should I add?
About 10% for simple houses and up to 15% for complex ones.
How do I find the area of a gable?
Half the base times the height.
What does installed vinyl siding cost?
Roughly $3 to $8 per square foot.