Paint coverage explained: why 400 sq ft a gallon rarely happens

Where the label figure comes from

A can of interior wall paint usually says it covers 350 to 400 square feet per gallon. That is a fair figure, measured honestly, but measured under ideal conditions: a smooth, sealed surface, the paint applied at the film thickness the manufacturer recommends, and nothing lost to the roller, the tray or the air. The figure is a property of the paint. What you get on your wall is a property of the paint, the wall and the way you apply it.

Most people who run short do so because they planned with the top of the range on a wall that was nowhere near ideal. The fix is not to distrust the label but to know which way each factor moves it.

The physics: volume solids and film thickness

Paint is solids (resin and pigment) carried in water or solvent. When the liquid evaporates, the solids stay as the dry film. A gallon is 231 cubic inches, and spread one thousandth of an inch (one mil) thick it would cover 1,604 square feet. Divide by the dry film thickness you want, multiply by the fraction of the paint that is solids, and you have the theoretical coverage.

A typical wall paint is about 40% solids by volume and is meant to dry to about 1.5 mils a coat. That gives 1,604 × 0.40 ÷ 1.5 = 428 sq ft per gallon in theory. With a roller, which delivers about 90% of the paint to the wall, the practical figure is 385 sq ft per gallon. That is the label range, and it is the best case.

This is the main reason a cheap paint covers less: it has fewer solids, so you need a thicker wet coat, or more coats, to build the same dry film. The volume solids figure is on the technical data sheet. The spread rate and film thickness calculator does the arithmetic, and wet film, dry film and volume solids goes deeper.

What eats into it on a real wall

Texture. A textured wall has more surface than its measured area. Orange peel and knockdown add a little; a heavy texture or a popcorn ceiling adds a lot, and the paint also has to fill the hollows. We apply a factor to the label coverage:

Porosity. Bare drywall paper, fresh joint compound, bare wood, masonry and old flat paint all soak up the first coat. The paint that disappears into the surface does nothing for colour or hiding. This is why a primer goes on new drywall: primer is cheaper than finish paint and seals the surface so the finish coats spread at their proper rate. It is also why primer data sheets print a lower coverage for new drywall than for previously painted walls.

Colour and hiding. Coverage and hiding are not the same. A gallon of deep red will spread over 400 sq ft perfectly well; it just will not hide what is underneath in one or two coats. Deep and saturated colours are mixed in bases with little white pigment, so they need more coats rather than a lower coverage figure. The coats calculator handles this, and dark colours and LRV explains why.

Application method. A roller loses about 5% to the roller cover, the tray and spatter. An airless sprayer loses far more to overspray: about 20% indoors, 35% outdoors and 40% for fine finishing, depending on the tip and the wind. Spraying a 1,000 sq ft job at two coats takes 7.14 gallons against 6.02 rolled, from the spray versus roll calculator. Spraying is faster; it is never more economical with paint.

The painter. A painter who rolls thin covers more area per gallon, and gets worse hiding and a less durable film. One who loads the roller heavily covers less. The 350 default assumes an even, proper coat.

Putting it together

Which figure to plan with

• Smooth, previously painted walls: 350 sq ft per gallon, the default in our wall paint calculator. Use 400 only if the can says so and the walls are genuinely smooth.
• Textured walls: apply the factor, or lower the coverage to about 300.
• New drywall: prime first, then plan the finish coats at 350.
• Ceilings: 400 on smooth, much less on texture; see textured ceilings and coverage.
• Exterior, masonry and stain: use the product’s own range, which is often far below wall paint; the coverage rates table lists them.

Reading the data sheet

The can label gives one coverage range. The product’s technical data sheet, which every major manufacturer publishes online, gives the figures behind it, and three of them are worth looking up before a large job:

• Spread rate, sometimes split by surface (“smooth” and “rough” or “porous”), in sq ft per gallon per coat. Use the rough figure for textured walls.
• Volume solids, as a percentage. Higher is more paint left on the wall per gallon. Compare it between two paints with the same label coverage before you decide which is better value.
• Recommended film thickness, wet and dry, in mils. If the data sheet asks for 4 mils wet and you roll 3, you will get the label coverage and less than the intended hiding.

Some data sheets also give coverage for the first coat on bare surfaces separately. Where they do, use it for the first coat and the normal figure for the second.

Checking your figure on the job

The best coverage figure is your own. After the first coat on the first wall, note how much paint is gone and how much area it covered. If a gallon did 300 sq ft rather than 350, scale the rest of the order before the store closes. It takes a minute and it is the only number that includes your wall, your roller and your hand.

Common mistakes

• Planning with 400 because the can says “up to 400”. “Up to” is the best case.
• Ignoring texture. Popcorn and heavy knockdown can halve the coverage of a ceiling paint.
• Skipping primer on bare surfaces to save money. The first finish coat soaks in and you buy more finish paint than the primer would have cost.
• Thinning paint to make it go further. It spreads further and hides less; you add a coat.
• Comparing coverage between brands without the volume solids. Two paints with the same label figure can differ by a third in solids.