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Exterior paint is bought by the gallon and spread by the square foot. The Exterior Paint Calculator works out the paintable wall area — perimeter × wall height, plus each gable triangle, minus a typical 15 sq ft per window and 20 sq ft per door — multiplies it by the number of coats and divides by the coverage on the can. BEHR PREMIUM PLUS Exterior lists 250 to 400 sq ft per gallon per coat; the calculator uses 350 for smooth or previously painted siding and 250 for rough or porous surfaces such as rough-sawn wood, cedar shakes, stucco and masonry, and shows the gallons at both ends of the range. A 40 × 30 ft house with 9 ft walls and two 6/12 gables has 1,295 sq ft to paint: two coats take 8 gallons on smooth siding (one 5-gallon bucket and three gallons) or 11 on rough siding. Add trim by the lineal foot: 300 ft of 6 in fascia and casing is 150 sq ft, about one more gallon.
Gallons = paintable area × coats ÷ coverage per gallon. BEHR PREMIUM PLUS Exterior lists 250–400 sq ft per gallon per coat; this calculator uses 350 for smooth or previously painted siding and 250 for rough or porous surfaces (rough-sawn wood, shakes, stucco, masonry). Paintable area = perimeter × wall height + gable triangles − windows (15 sq ft) and doors (20 sq ft).
Work out the wall area — perimeter × wall height, plus gables, minus windows and doors — multiply by the number of coats and divide by the paint's coverage. BEHR PREMIUM PLUS Exterior lists 250 to 400 sq ft per gallon. A 40 × 30 ft house with 9 ft walls and two 6/12 gables has about 1,295 sq ft to paint: two coats at 350 sq ft per gallon is 8 gallons, or 11 on rough siding.
Most exterior paints cover 250 to 400 sq ft per gallon per coat — BEHR PREMIUM PLUS Exterior gives exactly that range. Smooth, previously painted siding sits at the top; rough-sawn wood, cedar shakes, stucco and masonry soak up more and sit at the bottom.
Plan on two. The coverage on the can is per coat, and a second coat is what gives full hide and the film thickness the warranty assumes, especially when changing colour. One coat can be enough when repainting the same colour over sound paint.
Multiply the lineal feet of fascia, corner boards and casing by the face width in feet to get the area, then by the coats and divide by the coverage. 300 ft of 6 in trim is 150 sq ft — two coats is about 0.9 gallon, so buy a gallon.
Above about four gallons a 5-gallon bucket is usually cheaper per gallon and keeps the colour consistent. Box the paint — mix all the cans together — if you buy several, so the colour does not shift between walls.
Replacing the cladding instead? The siding calculator gives squares, vinyl cartons and fiber cement planks.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.