Understanding your result
In the area-to-paint direction, the headline is the paint for the area and coats, with the allowance, and the sub-line is the cans; the figures show the real coverage, the paint per coat and the cans. In the paint-to-area direction, the headline is the area the paint will cover at the coats you chose, the sub-line is the single-coat area, and the figures show the real coverage and both areas.
Both directions show the same three lines at the top of the table: the coverage on the can, the surface factor, and the real coverage per coat that the rest of the arithmetic uses. The paint-to-area direction adds the result as a length of 8 ft wall, which is the easiest way to picture an area: 149 sq ft is about 19 ft of wall, one long wall of a bedroom.
This is the general tool. The wall paint calculator applies the same arithmetic to a room with openings subtracted; the square footage calculator measures the area; the spread rate calculator derives the coverage itself from volume solids and film thickness; and the coats calculator decides how many coats to enter.
How we calculate this
The surface factor is the one non-obvious step. Manufacturers’ data sheets quote coverage as a range, with the high end for smooth, sealed surfaces and the low end for rough or porous ones; the low end is typically 70 to 85% of the high end. The factors here are that ratio, so a surface setting changes the coverage in the same proportion as the data sheet would. The coverage guide explains texture, porosity, colour and method one by one, and the coverage rates table lists the published ranges by product.
In metric mode the coverage is in square metres per litre and the conversion factor is exact: one square metre per litre is 40.746 sq ft per US gallon. The allowance applies only in the paint-for-area direction; in the other direction the figure is the area the paint covers if none is lost, and the note suggests keeping some back.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Coverage on the can | 350 sq ft per gallon (8.6 m²/L) | The lower end of interior wall paint data sheets (Sherwin-Williams, Benjamin Moore, Behr): 350–400 |
| Smooth surface factor | 1.0 | The data sheets’ smooth, sealed figure |
| Light texture factor | 0.85 | Site estimate between the smooth and rough figures |
| Rough or porous factor | 0.7 | The low end of the data sheets’ ranges as a share of the high end |
| Coats | 2 | Manufacturers’ recommendation for a colour change |
| Allowance | 5% | Roller and tray loss |
| Unit conversion | 1 m²/L = 40.746 sq ft/US gal | Exact, from 0.3048 m per ft and 3.785411784 L per gallon |
Assumptions last reviewed October 8, 2026.
The calculator does not model a first coat on bare surfaces that drinks far more than the second; for new drywall a primer is the cheaper answer, and the primer calculator has the coverage by surface. It does not model spraying, which adds overspray on top; the spray versus roll calculator does.
Two worked examples
500 sq ft of smooth wall, two coats
Previously painted drywall, paint rated at 350 sq ft per gallon, 5% allowance.
- Real coverage: 350 × 1.0 = 350 sq ft per gallon
- Per coat: 500 ÷ 350 = 1.429 gallons
- Paint: 1.429 × 2 × 1.05 = 3 gallons, buy 3 × 1-gallon cans
The same 500 sq ft of rough surface, one coat, no allowance, is 245 sq ft per gallon real coverage and 2.04 gallons: two gallons and a quart.
How far does what I have go?
One gallon of a paint rated at 350, on a lightly textured wall, two coats.
- Real coverage: 350 × 0.85 = 298 sq ft per gallon
- One coat: 1 × 298 = 298 sq ft
- Two coats: 298 ÷ 2 = 149 sq ft, about 18.6 ft of 8 ft wall
A feature wall, not a room. A 5-gallon pail at 400 on a smooth wall, by contrast, covers 2,000 sq ft once and 1,000 sq ft twice, about 125 ft of 8 ft wall.
Where to find your inputs
Area. From the square footage calculator, or length times height for each wall less the openings.
Paint on hand. The cans in the garage; a full gallon can of latex weighs about 11 to 12 lb, and the paint weight calculator converts a weight to gallons.
Coverage on the can. Printed on the label or the technical data sheet, usually as a range. Use the lower figure if unsure.
Surface. Look and touch. Sheen and smoothness mean smooth; orange peel or chalky flat paint mean light; bare drywall, heavy texture, raw wood or masonry mean rough.
Common mistakes
- Using the top of the can’s range. It is for a perfect surface; take the lower figure.
- Applying the texture twice. If you entered a textured-surface coverage from the can, set the surface to smooth.
- Forgetting the coats. A gallon that covers 350 once covers 175 twice.
- Mixing UK and US gallons. A UK data sheet’s gallon is 20% larger.
- Expecting the first coat on bare drywall to match the can. Prime it first.
- Ignoring the allowance. The roller and tray hold paint you cannot use.
Questions people ask
- How many square feet does a gallon of paint cover?
- Interior wall paint data sheets print 350 to 400 sq ft per gallon per coat on a smooth, sealed surface. On a lightly textured or porous wall the real figure is about 15% lower and on bare drywall, rough wood or masonry about 30% lower. One gallon at 350 covers about 150 sq ft at two coats on a lightly textured wall.
- How much area will 5 gallons cover?
- At 400 sq ft per gallon on a smooth surface, 2,000 sq ft in one coat and 1,000 sq ft in two, which is about 125 ft of 8 ft wall, the walls of a couple of medium rooms. On a rough surface the same pail covers 30% less.
- Why is real coverage lower than the can says?
- The can's figure is measured on a smooth, sealed test panel at the recommended film thickness. Real walls have texture, which adds surface; porosity, which soaks up the first coat; and rollers and trays, which hold paint. The surface factor here covers the first two and the allowance covers the third.
- What surface factor should I use?
- Smooth for previously painted drywall and primed trim. Light for orange-peel texture, flat old paint and lightly porous surfaces. Rough for bare drywall, heavy texture, rough-sawn wood and masonry. If the can gives its own figure for your surface, enter that as the coverage and choose smooth.
- How do I convert coverage to square metres per litre?
- Divide square feet per US gallon by 40.75. 350 sq ft per gallon is 8.6 m² per litre and 400 is 9.8. The calculator converts automatically when you switch to metric.
- Does the number of coats change the coverage?
- Not the coverage per coat, but the total. Two coats need twice the paint, so a gallon that covers 350 sq ft once covers 175 sq ft twice. The second coat often goes a little further than the first because the surface is sealed; the calculator treats them alike, which is the safe side.