Understanding your result
The headline is the exterior paint for the coats you chose, including the allowance, and the sub-line is the can list; at this scale it is usually 5-gallon pails. The four figures show the net wall area, the siding factor, the paint per coat and the cans.
The breakdown goes from the perimeter to the walls (perimeter times height times storeys), adds the gables, subtracts the openings, applies the siding factor to give the effective area, and then runs the usual coverage, coats, allowance and can lines. The effective area is the figure that explains the quantity: on stucco it is 40% more than the net wall area, which is why a stucco house takes a pail more than a smooth one.
Siding is only the biggest line of an exterior job. The exterior trim calculator counts the fascia, soffits, corners and casings, which are a different colour and often a different product; the garage door calculator does the door; and the stucco and masonry calculator replaces this one for an unpainted or chalky masonry wall, where the first coat drinks far more than a factor can express. The painter cost estimator uses the same wall area if you are comparing a quote.
How we calculate this
The walls are a rectangle wrapped around the house: the perimeter times the height. The height per storey is measured from the foundation to the eave and divided by the storeys; 9 ft per storey is typical for an 8 ft ceiling plus the floor structure. A gable is a triangle, half its width times its height from the eave line to the peak, and a house usually has two. Hipped roofs have no gables. The exterior measuring guide shows how to handle a split level, a bay and a dormer.
The siding factor is the one input other calculators do not have. Exterior paint data sheets quote coverage as a range, typically 250 to 400 sq ft per gallon, with the low end for “rough or porous” surfaces; contractor estimating guides express the same thing as a multiplier on the face area. The factors here are mid-points of those ranges, and the siding guide explains each one with the reasons. The coverage rates table has the published ranges by product.
Openings use the same standard sizes as every calculator on the site, from the standard sizes page, and the rounding to cans is the same arithmetic as the paint coverage calculator.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Coverage, smooth surface | 350 sq ft per gallon | Exterior acrylic data sheets (Sherwin-Williams Duration, Benjamin Moore Aura Exterior, Behr Premium Plus Exterior): 250–400 sq ft per gallon depending on surface; 350 is the smooth figure |
| Lap / clapboard factor | 1.1 | The overlapping edge adds about a tenth to the face area |
| Rough-sawn, shakes, shingles | 1.3 | The low end of manufacturers’ rough-surface coverage expressed as a multiplier |
| Stucco | 1.4 | Manufacturers quote 200–250 sq ft per gallon on stucco against 350 smooth |
| Brick, block | 1.5 | Masonry paints quote 150–250 on brick and block; the first coat on unpainted masonry takes more still |
| Wall height per storey | 9 ft (2.7 m) | 8 ft ceilings plus the floor structure |
| Openings | 20 sq ft per door, 15 per window | Standard sizes page |
| Coats | 2 | Manufacturers’ warranty condition for a full repaint |
| Allowance | 5% | Roller and brush loss; use 25–35% if spraying |
Assumptions last reviewed October 7, 2026.
The calculator does not model primer, which bare wood, patches and chalky old paint need (the primer calculator takes an area in its measured-area mode), the extra the first coat on unpainted masonry drinks, or overspray. It assumes the openings are standard sizes; a wall of glass or a double garage door should be counted as several windows or doors.
Two worked examples
A single-storey ranch on lap siding
160 ft perimeter, 9 ft walls, two gables 30 ft wide and 8 ft high, two doors and ten windows, clapboard, two coats at 350 sq ft per gallon.
- Walls: 160 × 9 × 1 = 1,440 sq ft; gables: 2 × ½ × 30 × 8 = 240 sq ft
- Openings: 2 × 20 + 10 × 15 = 190 sq ft; net 1,490 sq ft
- Effective: 1,490 × 1.1 = 1,639 sq ft
- Per coat: 1,639 ÷ 350 = 4.683 gallons
- Needed: 4.683 × 2 × 1.05 = 9.83 gallons, buy 2 × 5-gallon pails
Two pails with two-thirds of a quart over. If the store will not take back an unopened gallon, this is the right buy; the pail price per gallon is lower anyway.
A two-storey house in cedar shakes
180 ft perimeter, two storeys of 9 ft, two gables 28 × 7 ft, three doors and fourteen windows, rough-sawn shakes, two coats.
- Walls: 180 × 9 × 2 = 3,240 sq ft; gables 196 sq ft
- Openings: 60 + 210 = 270 sq ft; net 3,166 sq ft
- Effective: 3,166 × 1.3 = 4,116 sq ft
- Per coat: 4,116 ÷ 350 = 11.759 gallons
- Needed: 11.759 × 2 × 1.05 = 24.69 gallons, buy 5 pails
Shakes are nearly always sprayed and back-brushed, which adds a quarter to a third again: six or seven pails in practice, as the result’s note says. The same house in stucco, one coat as a same-colour refresh, would be 2,580 sq ft net, 3,612 effective and 10.84 gallons, two pails and a gallon.
Where to find your inputs
Perimeter. Tape or pace all the way round at the foundation, including bays and bump-outs, which add their own sides. If you have the plans, add the four sides. For an L-shaped house the perimeter is longer than a rectangle of the same floor area, which is the point of measuring it.
Storeys and height. Foundation to eave, measured with a tape from a ladder or a laser from the ground, divided by the storeys. A walkout basement with painted siding is a storey.
Gables. Width along the eave line, height from that line to the peak. Measure the height from the ground with a laser or estimate it from the roof pitch: a 6-in-12 pitch on a 30 ft gable is 7.5 ft high.
Siding. Look at it. Flat fibre cement and metal are smooth; wood or vinyl clapboard is lap; shakes and board-and-batten are rough; stucco and brick are themselves. Vinyl siding can be painted with the right paint and counts as lap.
Common mistakes
- Estimating from the floor area. Walls are roughly two to two and a half times the floor area for a single storey; measure them.
- Forgetting the gables. Two 30 × 8 gables are 240 sq ft, most of a gallon per coat.
- Using the smooth coverage on rough siding. Shakes take a third more; stucco 40% more.
- Counting the garage door as one door. It is 56 or 112 sq ft; count it as three or six.
- Skipping primer on bare or chalky areas. The topcoat peels there first. Prime them, and count the primer separately.
- Painting in the wrong weather. Most paints need 50 to 90 °F, a dry surface and no rain for 24 hours; the drying calculator shows what humidity does to the recoat time.
Questions people ask
- How much paint does a 2,000 sq ft house need?
- Floor area is a poor guide; the walls are what count. A single-storey house with a 160 ft perimeter, 9 ft walls and two gables has about 1,490 sq ft of net wall. On lap siding, which adds about 10%, two coats at 350 sq ft per gallon come to 9.83 gallons, so two 5-gallon pails.
- How do I measure the house for paint?
- Walk the perimeter with a tape (or pace it, at about 3 ft a pace), measure the wall height from the foundation to the eave, count the storeys, and measure the width and height of each gable. Count the doors and windows. The measuring guide covers bays, dormers and split levels.
- What does the siding factor do?
- It multiplies the net wall area by how much more paint the surface takes than a smooth panel, from the ranges on exterior paint data sheets. Smooth is 1.0, lap or clapboard 1.1, rough-sawn wood and shakes 1.3, stucco 1.4, brick and block 1.5. Change it if your can gives a figure for your surface.
- Do I subtract the windows outside?
- Yes, at 15 sq ft each, and doors at 20. A garage door is 56 sq ft for a single and 112 for a double, so count it as three or six doors. The exterior trim around the openings is painted separately and the exterior trim calculator counts it.
- How many coats outside?
- Two coats of topcoat over a sound, primed surface is what the manufacturers' warranties require. One coat only refreshes the same colour over paint in good condition. Bare wood, patches and chalky surfaces need primer first, which is a separate quantity from the primer calculator.
- Should I spray or roll the exterior?
- Spraying is faster on large plain walls and the only practical way on shakes and rough wood, but airless overspray adds 25 to 35% to the paint and needs masking. Rolling with cutting-in by brush uses the figure here plus the 5% allowance. The spray versus roll calculator puts the two side by side.