Exterior Paint Calculator

How to Calculate Exterior House Paint

Exterior paint projects are expensive mistakes when you run short mid-job. A second batch (dye lot) may not color-match perfectly, rain can ruin unprotected siding, and ladder rental or scaffolding rental clocks keep ticking. Coverage varies dramatically by surface type - smooth pre-primed fiber cement covers more than twice as much per gallon as bare stucco or unpainted brick. Getting the estimate right the first time is essential.

This calculator handles the six most common exterior surfaces (smooth, wood, fiber cement, vinyl, stucco, brick) with accurate coverage rates for each, plus automatic subtraction of doors and windows and separate calculation for gable ends and trim.

Wall Area Formula

The main wall area is the perimeter of the house times the wall height. For a rectangular 40x50 ft ranch with 9-foot walls, the perimeter is 180 ft and the wall area is 1,620 sq ft:

Gable Ends (Triangular Wall Sections)

Two-story and A-frame homes have triangular gable walls at each end. The area of each gable is half the base times the height from the eaves to the peak:

Subtract Doors and Windows

Typical opening areas:

• Entry door - 21 sq ft (3 ft x 7 ft)
• Garage door (single) - 63 sq ft (9 ft x 7 ft)
• Garage door (double) - 112 sq ft (16 ft x 7 ft)
• Standard window - 12-15 sq ft (3 ft x 4-5 ft)
• Picture window - 24-30 sq ft

Surface Coverage Rates

Body Paint vs Trim Paint

Most homes are painted in two colors: the main body and a contrasting trim color. These should be ordered in separate sheens:

When to Paint Exterior

Exterior painting is weather-dependent. Follow these conditions for best results:

• Air temperature - 50-90F (10-32C) during application AND overnight
• Surface temperature - same range; direct hot sun can heat siding 20F+ above air temp
• Humidity - below 70% ideal; above 85% paint cures poorly
• Rain forecast - no rain for 24-48 hours after application
• Wind - under 10 mph; wind blows debris into wet paint
• Dew point - apply at least 5F above the dew point

Typical Gallon Estimates by House Size

Prep Work Before Painting

1. Pressure wash - remove dirt, mildew, and loose paint at 1,500-2,500 PSI
2. Scrape loose paint - carbide scraper plus paint shaver for large peels
3. Sand edges - feather transitions between bare wood and painted areas
4. Spot prime - bare wood, stains, repairs, and drastic color changes
5. Caulk gaps - siding-to-trim joints, around windows and doors
6. Mask windows and adjacent surfaces - painter's tape + plastic sheeting
7. Apply body coat - work top to bottom, shaded side first
8. Apply second coat - 4+ hours later depending on temperature
9. Paint trim last - semi-gloss over satin body
10. Remove masking - while paint is still slightly soft to avoid pulling