A 40 by 28 foot house with 9 foot walls and two gable ends at a 6/12 pitch has 1,234 square feet of siding to paint once two doors and twelve windows are taken off. Two coats of Sherwin-Williams Duration on lap siding, at 275 square feet per gallon plus 10% waste, need 9.87 gallons: two 5-gallon pails.
How the exterior paint calculator works
The body of the house is a band as long as the perimeter and as tall as the walls. The gable ends are triangles on top of it, each half the gable width times its rise. Doors and windows come off, and the rest is divided by the spread rate for your siding.
Area = perimeter × wall height + gables × width × rise ÷ 2 − doors × 21 sq ft − windows × 12 sq ft
Gallons = area × coats ÷ sq ft per gallon × (1 + waste)
If you know the roof pitch rather than the gable height, the rise is half the gable width times the pitch over 12: a 28 foot wide gable at 6/12 rises 14 × 6 ÷ 12 = 7 feet. The door and window sizes (21 and 12 square feet) are the standard ones HomeAdvisor uses to work out paintable area.
Worked example: a 40 × 28 ft house
| Step | Numbers | Result |
|---|---|---|
| Walls | 2 × (40 + 28) ft × 9 ft | 1,224 sq ft |
| Two gable ends | 2 × 28 ft × 7 ft ÷ 2 | 196 sq ft |
| Doors and windows | 2 × 21 + 12 × 12 | 186 sq ft |
| Paintable area | 1,224 + 196 − 186 | 1,234 sq ft |
| Body paint, 2 coats, lap siding | 1,234 × 2 ÷ 275 × 1.10 | 9.87 gal |
| Trim, 200 ft of 5½ in boards | 91.7 sq ft × 2 ÷ 300 × 1.10 | 0.67 gal |
The body paint rounds to two 5-gallon pails and the trim to three quarts. Add a primer coat and the calculator adds 5.43 gallons, one pail and two quarts, at 250 square feet per gallon.
Coverage by siding type
Exterior paint covers less than interior paint because it goes on thicker and siding is rarely smooth. Sherwin-Williams lists Duration Exterior at 250 to 300 square feet per gallon, and Benjamin Moore lists Aura Exterior at 250 to 350, "depending on surface texture and porosity". Neither publishes a number for each siding type, so the calculator places your siding inside the range of the paint you pick:
| Siding | Where it sits in the range | Duration | Aura |
|---|---|---|---|
| Smooth panel or vinyl | top | 300 sq ft/gal | 350 sq ft/gal |
| Lap siding, wood or fiber cement | middle | 275 sq ft/gal | 300 sq ft/gal |
| Shingles or shakes | bottom | 250 sq ft/gal | 250 sq ft/gal |
| Stucco, brick or block | bottom, and check the can | 250 sq ft/gal | 250 sq ft/gal |
Treat stucco and brick as a floor, not an average. Sherwin-Williams gives only 80 square feet per gallon for its masonry coatings on porous, rough stucco. The same 1,234 square feet of stucco at 250 square feet per gallon already needs 10.86 gallons, two pails and a gallon; ask at the paint counter for the spread rate of the product you buy, and type it in with "From my can's label".
How many coats on the outside
Sherwin-Williams says to apply one coat of Duration on repaint work and two coats on bare surfaces. On stucco and masonry it calls for its Loxon masonry primer first. Choose "1 coat (repaint)" when you are going over sound paint in the same color, and two coats for a color change, bare wood or new siding.
Measuring a house for paint
- Perimeter: for a plain rectangle, length and width are enough. For an L-shaped house or one with bump-outs, switch to "Perimeter" and walk the outside wall by wall.
- Wall height: measure from the top of the foundation to the underside of the eaves. For a two-story house, it is both floors.
- Gables: count every triangular wall under a roof slope. A hip roof has none; a simple gable roof has two. If the pitch is unknown, measure the rise from the eave line to the peak.
- Trim: fascia, corner boards and door and window casing are painted in a separate, glossier paint. Add their length and face width; a 1x6 board is 5½ inches wide.
Lead paint on older houses
The EPA's renovation rule covers homes built before 1978. It generally does not apply to homeowners working on their own home, but lead dust from scraping old siding is just as toxic: test first, and use lead-safe methods if it is present.
Next steps
- Price the job with the painting cost calculator, which takes this paintable area and your siding.
- Painting the rooms too? The paint calculator does walls, ceilings and trim room by room.
- Replacing rather than painting? The siding calculator and vinyl siding calculator count squares and panels.
- Not sure of the gable pitch? The roof pitch calculator turns rise and run into x/12.
All paint and wall tools are collected in drywall, paint and walls.