Paint Calculator

How Paint Coverage Is Calculated

Every paint estimate starts with wall area, and wall area starts with the perimeter of the room rather than its floor area. Add up the length of all four walls, multiply by the ceiling height, and that is the gross paintable surface. A 12 ft by 14 ft bedroom has a perimeter of 52 feet, and at an 8 ft ceiling that is 416 square feet of wall before anything is taken out.

Those deductions matter more than people expect. The same 12 ft by 14 ft bedroom with one door and two windows loses 21 plus 30, which is 51 square feet, bringing the net wall area down to 365 square feet. On a small bathroom with a door, a window and a large vanity mirror, the deductions can be a fifth of the total.

Turning square feet into gallons

One US gallon of interior wall paint covers about 350 to 400 square feet in a single coat on a smooth, primed, previously painted surface. Use 350 for planning, because 400 is a laboratory figure that assumes a perfectly even film and no waste in the tray, on the roller cover or on the brush.

For the example bedroom at 365 square feet with two coats, that is 730 square feet of coverage, divided by 350, which is 2.09 gallons. Round to a 2 gallon purchase plus a quart, or simply buy 3 gallons if there is trim and a closet to cover as well.

Coverage drops on rough or absorbent surfaces. Knockdown or orange peel texture can cut coverage by 10 to 20 percent because the film has to fill the profile. Bare drywall, fresh joint compound, raw wood and old flat paint all soak up more. Smooth plaster and previously enamelled surfaces run at the top of the range.

How Many Coats the Job Needs

Two coats is the standard, and it is the standard for a reason. The first coat does the covering and the second coat evens out the sheen and the film thickness. A single coat almost always looks patchy in raking light, even when the color is identical, because roller pressure and overlap vary across a wall.

• Two coats over an existing painted wall going to the same color or a similar shade in the same family.
• Three coats over bare drywall, or when making a big color change, for example a deep red going to off white, a dark navy going to pale grey, or any strong accent wall being neutralized.
• Three coats when using a saturated color such as a true red, a bright yellow or a deep teal, since those pigments have poor hiding power regardless of quality.

Substituting a primer coat for one of the finish coats is usually the cheaper path. A gallon of primer costs less than a gallon of quality finish paint, blocks the substrate better, and gives the topcoat a uniform surface to sit on.

When Primer Is Genuinely Needed

Primer is not required on every job. Repainting a clean, sound, previously painted wall in a similar color does not need it, and most quality paints today carry enough binder to bond directly. There are four situations where skipping primer causes real problems.

Bare drywall and fresh joint compound

New drywall paper and dried joint compound absorb at wildly different rates. Paint straight onto them and every taped seam and screw dimple shows through as a duller patch, a defect called flashing. A drywall primer sealer, or a full skim of PVA primer, equalizes the porosity so the finish coat dries to one uniform sheen.

A large color change

Going from dark to light takes fewer finish coats if a grey tinted primer goes on first. Most paint stores will tint primer toward the finish color at no cost. A mid grey primer under a bright white can save an entire coat of expensive finish paint.

Stains and odors

Water stains, smoke damage, tannin bleed from knots in pine, crayon, marker and nicotine will all bleed through water based paint no matter how many coats go on. These need a stain blocking primer, either shellac based or oil based. Water based stain blockers work on light water stains but not on heavy tannin or smoke.

Glossy or slick surfaces

Old oil based enamel, previously varnished trim, glossy tile and laminate all need either a bonding primer or a thorough scuff sand followed by primer. Latex paint applied over aged gloss enamel will peel off in sheets months later, often taking a fingernail test to reveal it before the failure is visible.

Sheen Levels and Where Each Belongs

Sheen is the amount of light the dry film reflects, and it controls both how washable the paint is and how much of the wall's imperfection it advertises. Higher sheen is tougher and easier to clean, but it highlights every dent, patch and roller mark.

Flat and matte

Flat has almost no reflectance and hides surface defects better than anything else, which makes it the standard for ceilings and for older walls with a lot of patching history. Traditional flat paint is difficult to clean and burnishes when scrubbed. Modern matte and washable flat products close much of that gap and are a reasonable choice for adult bedrooms and formal living rooms.

Eggshell and satin

Eggshell has a slight low luster and satin is a step brighter. This pair covers most of a house: hallways, family rooms, kids' bedrooms, dining rooms and any wall that gets touched. Satin is the usual pick for bathrooms, laundry rooms and kitchen walls because it resists moisture and wipes down without polishing to a shine.

Semi gloss

Semi gloss is the standard for interior trim, doors, window casings, baseboards and cabinets. It is hard, it cleans well, and the contrast in sheen against eggshell walls is what makes trim read as trim. It is also used on bathroom walls where moisture is severe, though it will show every drywall flaw.

High gloss

High gloss is a specialty finish for front doors, furniture, railings and accent millwork. It is the most durable and the least forgiving. Any surface taking high gloss needs to be filled, sanded and primed to near furniture standard, because the finish acts like a mirror.

Cutting In

Cutting in is the brushed band of paint at the ceiling line, in the corners, around trim and along baseboards where a roller cannot reach. Plan on a band 2 to 3 inches wide. A 2.5 inch angled sash brush is the standard tool, and a good one holds enough paint to run 3 to 4 feet of line without reloading.

The sequence matters. Cut in one wall, then roll that wall while the cut line is still wet, so the brushed and rolled films blend. Cutting the whole room first and rolling afterward leaves a visible picture frame effect where the brushed texture dried before the roller reached it. Roll as close to the cut line as the roller will safely go, ideally within half an inch, so the brush texture band stays narrow.

Cutting in consumes paint at a different rate than rolling. A room needs roughly a quart of paint for cut in work per coat on an average bedroom, and that is already inside the 350 square foot per gallon figure, but it is worth knowing when a job is being split between two people from two cans.

Ceilings and Trim Are Separate Calculations

Ceiling area is the room's floor area, length x width, with no deductions unless there is a large skylight. The 12 ft by 14 ft bedroom has a 168 square foot ceiling, which at two coats is 336 square feet, just under a gallon. Ceiling paint is a different product from wall paint, formulated dead flat and with more open time so lap marks do not set before the roller returns.

Trim is measured in linear feet and converted roughly. Baseboard, casing and crown together typically use about 1 gallon per 300 to 400 linear feet per coat for standard profiles. Doors take about a quarter of a quart per side per coat. Because trim paint is usually a semi gloss enamel in a different base, it must be bought separately, and the quantity is small enough that a quart often covers an entire room's trim for two coats.

Room Size Reference

The table below assumes an 8 ft ceiling, one door, two windows, and two coats of wall paint at 350 square feet per gallon. Ceiling gallons are listed separately at two coats.

Add 12.5 percent to the wall figures for a 9 ft ceiling and 25 percent for a 10 ft ceiling, since the perimeter stays the same and only the height changes.

Practical Points That Change the Order

• Buy the whole job at once. Paint is tinted in batches and two cans mixed weeks apart can differ slightly. If more than one gallon is needed, box them, which means pouring all the cans into a single 5 gallon bucket and stirring, so every wall gets identical color.
• Five gallon pails are cheaper per gallon and worth it once a job passes about 4 gallons, which is roughly a whole floor of an average house.
• Keep half a quart labelled for touch ups, and write the room, the color code, the sheen and the date on the lid.
• Temperature and humidity matter. Most latex paints want 50 to 85 degrees Fahrenheit and below about 70 percent relative humidity. Cold or damp conditions slow the cure and can leave the film soft for weeks.