How the exterior paint calculation works
- Walls. Multiply the distance around the house by the wall height from the ground (or the top of the foundation) to the eaves.
- Gables. Each triangular gable end adds half its width times its height from eaves to peak.
- Openings. Subtract about 20 sq ft for each door and 15 sq ft for each window.
- Coats and texture. Multiply by the coats and divide by the label coverage, reduced for textured or rough surfaces.
gallons = (perimeter × height + gables × ½ × width × rise − openings) × coats ÷ (coverage × texture factor)
Worked example
A 40 × 30 ft single-story house has a 140 ft perimeter. With 10 ft walls that is 1,400 sq ft. Two gables, each 30 ft wide and 6 ft tall, add 180 sq ft, for 1,580 sq ft. Two doors and twelve windows take off 220 sq ft, leaving 1,360 sq ft. Two coats at 350 sq ft per gallon need about 7.8 gallons: buy 8, or two 5 gallon buckets. In metric that is about 29.4 liters, or three 10 L cans.
The same house with rough-sawn wood siding needs about 9.7 gallons. With stucco it needs about 13 gallons.
Surface texture and coverage
| Surface | Coverage factor | 350 sq ft/gal becomes |
|---|---|---|
| Smooth lap siding, fiber cement, smooth panels | 1.0 | 350 sq ft/gal |
| Rough-sawn wood, wood shingles, grooved panels | 0.8 | 280 sq ft/gal |
| Stucco, brick, concrete block, split shakes | 0.6 | 210 sq ft/gal |
These factors are rules of thumb. Rough and porous surfaces have more real surface area and soak up paint, and the first coat on bare masonry or weathered wood can use even more. Some labels give separate coverage figures for rough surfaces; when they do, enter that figure and choose smooth.
Measuring the house
- Walk around the house with a long tape or a measuring wheel and add up the wall lengths. Bump-outs and bay windows add perimeter.
- Measure the wall height at one corner. On a two-story house, it is often 18 to 20 ft to the eaves; measure from a window you can reach rather than guessing.
- Count only the gable triangles that get the same paint. Gables with different siding can be a separate calculation.
- Garage doors are large: count a single garage door as several standard doors, or work out its area and subtract it.
Trim, primer and second coats
Trim, fascia, soffits and doors usually take a different color and sheen, so estimate them separately. Bare wood, new fiber cement that is not factory primed, and repaired areas need primer first. Two finish coats are the norm for a color change or for weathered surfaces; see how many coats of paint for when one is enough.
Buying tips
- Buy all the paint at once and from the same batch where possible. Even custom-mixed colors can vary slightly between mixes.
- For a large wall, pour several gallons into one big bucket and stir them together, called boxing. It keeps the color even from one can to the next.
- Primer has its own coverage rate. Run the calculator again with 1 coat and the primer's coverage for the areas that need it.
- Keep a labeled quart for touch-ups, stored where it will not freeze.
Safety
- Ladder work is where most painting injuries happen. Set extension ladders at about 1 ft out for every 4 ft of height, keep three points of contact, and stay clear of overhead power lines.
- In the US, homes built before 1978 may have lead paint. Do not dry-sand or power-wash old paint until you know; use lead-safe practices or a certified contractor.
- Scaffolding or a professional crew is the safer choice for two-story walls and steep ground.
Painting inside too? The interior paint calculator handles rooms and ceilings.
Frequently asked questions
How many gallons of paint do I need for the outside of a house?
A 40 × 30 ft single-story house with 10 ft walls, two gables, two doors and twelve windows has about 1,360 sq ft to paint. Two coats at 350 sq ft per gallon need about 7.8 gallons, so buy 8.
Do I subtract windows and doors from exterior paint?
Yes. Taking off about 20 sq ft per door and 15 sq ft per window keeps the estimate close. Large garage doors should be measured and subtracted on their own.
How much more paint does stucco need?
As a rule of thumb, rough surfaces like stucco cover about 40 percent less than smooth siding, so 350 sq ft per gallon becomes about 210. The first coat on bare stucco may need even more.