How much paint to paint a house exterior

The short answer

Measure around the outside of the house to get its perimeter, multiply by the wall height, add the triangular gable ends, and subtract the doors and windows. Multiply the result by two coats and divide by the coverage on the paint label, which for exterior house paint on smooth siding is typically 350 to 400 sq ft per gallon. Wood lap, rough wood, shingles, stucco and brick take more paint, so the coverage is multiplied by a siding factor below 1 for those surfaces. The exterior paint calculator runs these steps, applies the siding factor and turns the answer into 5-gallon buckets, gallons and quarts.

As a rough scale, 1,500 sq ft of paintable siding needs about 8.6 gallons for two coats on smooth siding at 350 sq ft per gallon, and about 12.2 gallons on rough-sawn wood or shingles. The worked example below shows where those numbers come from. Trim, soffits, doors and shutters are usually a different color and product, so estimate them separately.

Measure the walls

Walk around the house with a long tape measure and record the length of every wall at ground level. For a plain rectangular house, two measurements are enough: the perimeter is 2 × (length + width). For an L-shaped house or one with bump-outs, add up every wall segment instead. A sketch of the footprint with each length written on it is the easiest way to avoid missing a wall or counting one twice.

Next, find the wall height from the foundation line to the eaves, where the siding stops under the roof overhang. Wall heights vary with ceiling height, floor framing and how high the foundation sits, so measure rather than assume. If you cannot reach, count siding courses: measure the exposure of one board, count the boards from bottom to top and multiply. Perimeter × height gives the rectangular wall area.

If the house is built into a slope, one side has more exposed foundation and a taller wall. Measure that side separately and only include the foundation if you intend to paint it. The guide to measuring walls for paint covers splitting awkward shapes into simple rectangles.

Add the gables

On a house with a gable roof, the end walls continue above the eaves in a triangle up to the roof peak. That triangle is siding too, and it is easy to forget. Its area is ½ × base × height, where the base is the width of the wall at eave level and the height is the vertical distance from the eave line to the peak. You can estimate the peak height from the ground with a laser measure, or count siding courses in the triangle as you did for the walls.

On a rectangular house the base of each gable is the house width, the short side under the gable ends. A 30 ft wide end wall with a gable rising 8 ft adds ½ × 30 × 8 = 120 sq ft. A house with gables at both ends adds that twice. Hip roofs have no gable triangles, and dormers add small walls of their own; measure each dormer face as a rectangle or triangle and add it in.

Subtract doors and windows

Doors and windows are not painted with the siding color, so take their area off. Measuring each opening is the most accurate approach. For a quick estimate, the exterior paint calculator uses standard allowances of 21 sq ft per door (3 × 7 ft) and 15 sq ft per window (3 × 5 ft). A double garage door is about 112 sq ft, so count it as five doors or measure it.

Do not over-subtract. Window and door casings, sills and corner boards still need paint, usually the trim color, and the extra cutting-in around each opening uses some of the paint you saved. If the house has only a few small windows, leaving them in the estimate gives a small margin rather than a meaningful error.

How siding type changes coverage

The coverage printed on a can is measured on a smooth surface. Purdue Extension's guidance on finishing exterior wood puts a gallon of house paint at about 400 sq ft, and CompositePaint's exterior guide gives 350 to 400 sq ft per gallon for smooth lap or vinyl siding. Rough or textured surfaces have more real surface area than their measured outline and soak up more paint, so the same gallon covers less.

CompositePaint puts fiber cement and T1-11 at 300 to 350 sq ft per gallon, rough-sawn cedar at roughly 250 against about 350 for smooth cedar, stucco at about 25 percent more paint than a smooth estimate, and brick and block as low as 200 sq ft per gallon because the mortar joints hold paint. The exterior paint calculator turns those figures into the factors in the table: it multiplies the label coverage, 350 sq ft per gallon by default, by the factor for your siding. If your own product label gives a range, start from its smooth-surface figure.

Primer and the number of coats

Bare wood needs primer under latex house paint. Purdue Extension's guidance is to prime whether the topcoat is oil-based or latex, and for woods that bleed water-soluble extractives, such as cedar and redwood, to use a good oil-based or alkyd primer or a latex primer designed for those woods. CompositePaint gives the same advice for cedar: prime bare boards with a dedicated stain-blocking primer. Benjamin Moore's Fresh Start Deck & Siding Primer, an alkyd primer aimed at tannin-rich woods and weathered or chalky surfaces, lists 450 to 550 sq ft per gallon. Spot-prime bare patches where old paint has been scraped off; the guide on when to use primer explains the other cases.

Plan for two finish coats. According to Purdue, one topcoat over a properly applied primer should last 4 to 5 years, while two topcoats can last up to 10. One coat is reasonable only when you are repainting sound siding in the same color with a high-hiding paint, and even then the sunny south and west walls benefit from a second coat. See how many coats of paint you need for more on coats.

If the house was built before 1978, assume the old paint may contain lead until it has been tested. The EPA notes that about three quarters of US homes built before 1978 still contain some lead-based paint and that work disturbing painted surfaces can create dangerous lead dust. Contractors paid to do that work must be lead-safe certified.

Worked example

Take a single-story house 40 ft long and 30 ft wide with 10 ft walls and a gable roof whose two gable ends each have a 30 ft base, the house width, and rise 8 ft to the peak. The perimeter is 2 × (40 + 30) = 140 ft, so the rectangular walls cover 140 × 10 = 1,400 sq ft. Each gable adds ½ × 30 × 8 = 120 sq ft, and two gables add 240 sq ft, for a gross total of 1,400 + 240 = 1,640 sq ft.

The house has 2 doors and 12 windows. Using the 21 and 15 sq ft allowances, the openings come to 2 × 21 + 12 × 15 = 42 + 180 = 222 sq ft, leaving 1,640 − 222 = 1,418 sq ft of siding to paint.

With vinyl or smooth lap siding and paint labeled 350 sq ft per gallon, two coats need 1,418 × 2 ÷ 350 = 2,836 ÷ 350 = 8.1 gallons, rounded to one decimal place as the calculator does. That is one 5-gallon bucket, three gallons and one quart. With wood lap siding, coverage is 350 × 0.9 = 315 sq ft per gallon and the same two coats need 2,836 ÷ 315 = 9.0 gallons: one 5-gallon bucket and four gallons. With rough-sawn wood or shingles, coverage is 350 × 0.7 = 245 sq ft per gallon, and 2,836 ÷ 245 = 11.6 gallons: two 5-gallon buckets and two gallons.

The same walls in stucco, at 350 × 0.8 = 280 sq ft per gallon, need 2,836 ÷ 280 = 10.1 gallons: two 5-gallon buckets and one quart. In brick, at 350 × 0.6 = 210 sq ft per gallon, they need 2,836 ÷ 210 = 13.5 gallons: two 5-gallon buckets, three gallons and two quarts.

If all of that siding were new, bare wood, add one coat of primer first. At 450 sq ft per gallon, 1,418 ÷ 450 = 3.15 gallons, which rounds to 3.2: three gallons and one quart. Try your own measurements in the exterior paint calculator.

Sources

• Purdue Extension, NCR-135: Finishing exterior wood (priming, two topcoats, coverage per gallon)
• CompositePaint, Exterior paint calculator: coverage by siding, door and window allowances
• CompositePaint, Cedar siding paint calculator: rough-sawn vs smooth cedar, priming bare cedar
• Benjamin Moore, Fresh Start Deck & Siding Primer product page (tannin-rich woods, 450 to 550 sq ft per gallon)
• US EPA, Lead Renovation, Repair and Painting Program