How to calculate exterior square footage for siding, paint, and roofing bids

Most bid disputes trace back to the same root: the contractor and the homeowner measured different things. One counted gross wall area; the other subtracted windows and doors. One used a flat footprint; the other accounted for pitch. Getting the number right before the job starts is how you protect your margin and your reputation.

Here's the math, trade by trade.

Siding

Siding is billed in square feet of wall surface after deductions for openings (windows and doors). A standard approach:

1. Measure the perimeter of the house at grade.
2. Multiply by the wall height (eave to foundation) to get gross wall area.
3. Subtract 15 sq ft per standard window and 21 sq ft per standard door.
4. Add a waste factor: 10% for straight runs of horizontal lap; 15% for vertical or diagonal patterns; up to 20% around complex corners or dormers.

For a simple 1,800 sq ft ranch with 8-foot walls, a 140-foot perimeter, 8 windows, and 2 doors:

• Gross wall area: 140 × 8 = 1,120 sq ft
• Deductions: (8 × 15) + (2 × 21) = 162 sq ft
• Net: 1,120 - 162 = 958 sq ft
• With 10% waste: 958 × 1.10 = 1,054 sq ft ordered

Complex rooflines, gables, and dormers add surface area that the perimeter method misses. That's where photo measurement earns its keep: the algorithm sees the whole face, not just the base perimeter.

Painting

Painting square footage is similar to siding with one difference: painters rarely deduct small windows and do deduct large sliding doors. A common rule of thumb is to deduct any opening over 100 sq ft and ignore the rest.

Two coats is the default. Rough surfaces (stucco, old wood with deep texture) can require 20-30% more paint per square foot, so factor that into your labor estimate as well as materials.

For primer plus two finish coats on 1,000 sq ft of net wall area, you're typically looking at 4-5 gallons per coat, or 12-15 gallons total depending on surface texture and the specific paint's coverage rate (always check the manufacturer's spec sheet).

Roofing

Roofing is sold in squares (1 square = 100 sq ft of roof surface). The tricky part is that roof surface is always larger than the footprint because of pitch.

The conversion factor by pitch:

Pitch (rise:run)	Multiplier
4:12	1.054
5:12	1.083
6:12	1.118
7:12	1.158
8:12	1.202
9:12	1.250
10:12	1.302
12:12	1.414

For a 2,000 sq ft footprint at 6:12 pitch:

• Roof surface: 2,000 × 1.118 = 2,236 sq ft
• In squares: 22.36 squares
• Waste factor: 10-15% for a simple gable, up to 20% for complex hip roofs with multiple valleys
• Ordered: 22.36 × 1.12 = 25.0 squares

Hips, valleys, ridges, and skylights each add material and labor. A "simple" roof is rare in practice.

The measurement gap that costs you

The numbers above assume you already know the wall dimensions accurately. That's where most bids go wrong. Pacing off a perimeter with a tape measure gives you the footprint but misses dormers, bump-outs, and multi-story sections. A street-level estimate from memory or a rough sketch is worse.

A photo-based measurement closes the gap. You see the actual surface: every gable, every shed dormer, every section change. The square footage that comes out maps to what you'll actually install or paint, not an approximation from the ground.

A note on gutters and windows

Gutters are measured in linear feet along the eave line. Windows are a separate scope: count by unit and type (double-hung, casement, picture, bay), not by square footage. Measure rough opening width and height; that's what the installer needs.

When you're quoting multiple trades on the same property, a single exterior measurement session covers all of them. Same photo, same dimensions, different line items.