Exterior Square Footage Calculator

For a single-story rectangular house, exterior wall area = perimeter × wall height. A 40 × 30 ft house with 9 ft walls has a perimeter of 140 ft × 9 ft = 1,260 sq ft of wall.

Add gable triangles for a pitched roof. A 30-ft-wide gable end with a 7/12 pitch has a peak height of (30 ÷ 2) × (7 ÷ 12) = 8.75 ft. Triangle area = (30 × 8.75) ÷ 2 = 131 sq ft per gable. Two gables = 262 sq ft to add.

Dormers, bays, and bump-outs each get measured as their own additional wall section. Eave overhangs do not get sided but are included if you're painting.

For siding and stucco, subtract garage doors and any opening larger than 25 sq ft. For paint, subtract all door and window openings (you only paint the trim around them, calculated separately as linear feet).

A standard double garage door is 16 × 7 = 112 sq ft. A standard window is 3 × 5 = 15 sq ft. A typical single-story 2,000 sq ft house with a garage has 100-150 sq ft of openings to subtract.

Exterior square footage is the surface area of all outside walls of a building — different from house square footage (interior livable space) and from siding area (which subtracts windows and doors). The right number depends on what you're measuring for.

For siding orders: perimeter × wall height, minus large window and door openings.

For exterior paint: perimeter × wall height, minus windows and doors over 10 sq ft.

For insulation: perimeter × wall height, minus only window areas (insulation goes around door frames).

For exterior cleaning/power washing: perimeter × wall height, including everything.

For curb appeal: gross perimeter × wall height including all features (the visual size).

Worked example: a 40 × 30 ft two-story house has 140 ft perimeter × 16 ft total height = 2,240 sq ft of gross exterior wall. Add gable triangles (2 × 90 sq ft = 180 sq ft). Total exterior = 2,420 sq ft.

Different house styles have very different exterior square footage even with the same floor plan:

Single-story (1,500 sq ft footprint): perimeter (150 ft) × 8 ft wall = 1,200 sq ft, plus 2 gables (~180 sq ft) = 1,380 sq ft exterior.

Two-story (same 1,500 sq ft footprint): perimeter (150 ft) × 16 ft total height = 2,400 sq ft, plus 2 gables = 2,580 sq ft exterior.

Split-level: typically 70% more exterior area than equivalent two-story.

1.5-story with dormers: add 50-100 sq ft per dormer.

Houses with covered porches, attached garages, and bonus rooms have additional exterior wall area for each attached structure.

Eave overhangs and rake overhangs add 10% to exterior surface area for typical residential roofs. Modern minimalist designs with no overhangs (or 'flat-roof' houses) have less.

Each exterior treatment has different square-foot requirements:

Exterior paint: 1 gallon covers ~275 sq ft per coat on smooth siding, ~200 sq ft on rough/textured. Two coats for new paint, one coat for recoating same color. A 2,400 sq ft house exterior: 2,400 × 2 ÷ 275 = 17.5 gallons. Buy 18-20 gallons.

Power washing service: $0.10-0.40 per sq ft depending on whether mildew/stain removal is included. A 2,400 sq ft house: $240-960.

Pressure washing the deck and walkway: typically priced separately, $0.20-0.50 per sq ft.

Exterior wood sealer (decks, fences, doors): 1 gallon covers ~150-250 sq ft on rough wood. Apply every 2-3 years.

House washing chemicals (sodium hypochlorite solution for mildew): $20-40 per gallon, covers ~300 sq ft when sprayed.