How to use this calculator
Enter the footprint of the house in feet — the long side and the short side — then the wall height to the eave. Pick the siding type, which sets the coverage figure. Add the gable ends and count the doors and windows.
The result is body paint, trim paint and primer as separate lines, because they are separate products at different prices and you will buy them on different days.
Measuring a house without a ladder
You do not need to climb anything. Pace the perimeter: a normal stride is close enough to 3 feet, and you only need the number to within a foot.
For height, count the siding courses and multiply by the exposure — the height of one course to the weather, which you can measure at ground level. A course count of 24 at 8 inches is 16 feet. A vinyl panel usually carries more than one course, so measure one panel before you count. If the siding is stucco or brick, measure a window and count how many window heights fit up the wall.
A single-storey ranch is typically 9 to 10 feet to the eave. A two-storey is 18 to 20. Take the larger figure — paint left over is cheaper than a second trip to the store on a scaffolding rental day.
Siding texture changes coverage
| Surface | Coverage per gallon | Gallons per 2,500 sq ft, two coats |
|---|---|---|
| Smooth lap or fibre cement | 350 sq ft | 14.5 |
| Vinyl siding | 325 sq ft | 15.5 |
| Stucco | 200 sq ft | 25 |
| Rough sawn wood or cedar shakes | 180 sq ft | 28 |
| Brick or masonry | 175 sq ft | 29 |
The reason stucco doubles the paint is surface area. A flat measurement of 100 square feet of rough stucco has perhaps 150 square feet of actual surface once you follow every ridge and pit. The same applies to cedar shakes and to any deeply grooved siding pattern.
The starting point for those figures is the manufacturer’s own: Sherwin-Williams says a gallon typically covers 350 to 400 square feet and that texture changes it. Smooth siding here uses 350, the low end; the lower figures for rough surfaces are this site’s estimates. The primer line is one coat at the same coverage as the paint, while Sherwin-Williams puts primer at 200 to 300 square feet a gallon, so on bare smooth siding buy more primer than the line shows.
Weather sets the schedule more than the paint does. The can states the temperature range for applying it, and the overnight low matters as much as the daytime high: dew forming on a coat that has not cured will leave a chalky bloom that has to be washed off and recoated. Check the low, not just the forecast temperature, and stop painting three hours before sunset in autumn.
Prep is the line item people leave out of the budget entirely. Washing, scraping, sanding feathered edges and caulking every joint takes longer than the painting on any house over ten years old — budget two days of prep for every one day of paint, and a tube of exterior-grade caulk per 40 linear feet of trim joint.
Gables and dormers
A gable is a triangle: half the width times the height. The calculator asks only for the width and takes the height as 30% of it, so a 28 foot wide gable rises 8.4 feet and comes to 117.6 square feet — small on its own, but two of them plus two dormers is over 300 square feet, which is a full gallon per coat.
Dormers are the part people forget completely. Each one has two small walls, a face and often a return, and they are the hardest surfaces on the house to reach. Count each dormer as roughly 60 square feet of body and 20 linear feet of trim.
Soffit and fascia are usually trim colour. Fascia is linear: the perimeter plus the rake lengths. Soffit is the perimeter times the overhang depth, usually 1 to 2 feet.
Common mistakes
- Estimating from the house square footage. A 2,000 square foot two-storey on a 40 by 25 foot footprint has about 2,306 square feet of siding in the calculator, and a different footprint gives a different number. Floor area and wall area are unrelated numbers.
- No primer on bare wood. Any board you replace, any spot where the old paint failed to the substrate, and every cut end needs primer. Paint over bare cedar will bleed tannin within one season.
- Painting in the sun. Follow the shade around the house. Paint applied to a wall at 120 degrees skins over before it levels and fails early.
- Spraying without back-rolling. Spraying alone lays the paint on the surface; back-rolling works it in, and on rough siding that is what gets paint into the texture.
- Under-ordering trim paint. Trim takes two coats of a more expensive product and there is more of it than you think — the linear foot method is in the baseboard and trim measuring guide.
- Wrong number of coats. One coat over a similar colour on sound paint is defensible outside; anything else is two. See how many coats of paint.