How much paint you need for a room, with the math worked out

The formula in four steps

1. Measure each wall's length and height in feet, multiply them, and add the walls together.
2. Subtract the big openings if you want a tighter number: doors, large windows, a fireplace.
3. Divide by the coverage printed on the can, in square feet per gallon.
4. Multiply by the number of coats. Benjamin Moore recommends at least two for walls.

Measure in feet and round up to the next half foot. A tape measure and a pencil are all you need, and you can do the whole room in the time it takes to find a calculator.

A worked example

Take a 12 by 14 foot bedroom with 8 foot ceilings, one door and two windows.

• Perimeter: 12 + 14 + 12 + 14 is 52 feet.
• Wall area: 52 times 8 is 416 square feet.
• Openings: one door at about 20 square feet (3 feet by 6 feet 8 inches) and two windows at 12 square feet each (3 by 4 feet). 416 minus 20 minus 24 is 372 square feet.
• Two coats: 372 times 2 is 744 square feet of paint to apply.
• At 350 square feet per gallon: 744 divided by 350 is 2.1 gallons.

Two gallons would leave you about 44 square feet short, so you'd buy two gallons and a quart, or a third gallon if you'd like a cushion. How much the coverage number matters shows up in this table:

The same method for other common bedroom and living room sizes, assuming 8 foot ceilings, one door and two windows, 350 square feet per gallon and two coats on the walls:

• 10 by 12 feet: about 1.8 gallons.
• 12 by 12 feet: about 1.9 gallons.
• 12 by 14 feet: about 2.1 gallons.
• 14 by 16 feet: about 2.5 gallons.

A room with more windows or a wide opening to the next room needs a little less. Fresh, bare drywall is porous and soaks up paint, so plan on the low end of the coverage range there.

Why coverage is a range

Paint makers don't print one number because a gallon covers a different area on different surfaces. Dunn-Edwards lists "practical coverage per coat" on its product data sheets, and adds that it depends on surface conditions and application techniques. One of its flat interior and exterior paints is listed at approximately 300 to 400 square feet per gallon. EPA's Wall Paint Exposure Model, a government tool for estimating exposure to paint emissions, assumes 400 square feet per gallon, citing paint container labels.

Rough or textured walls and bare, porous surfaces take more paint per square foot than smooth, previously painted ones, so plan on the low end of the range for those. The number also assumes the paint goes on at the thickness the maker recommends. Spreading it thinner stretches the gallon on paper, but it isn't what the number promises.

The label on your can is the number to trust. If you can't find one, 350 is a sensible middle.

Primer covers less. One maker's data sheet puts a multi-surface primer at about 225 to 275 square feet per gallon, below the 300 to 400 for its flat wall paint, and primer is its own coat, not part of the paint count. Our post on whether you need primer covers when it's worth it, and how many coats of paint covers how it changes the total.

Ceilings, trim and doors

Figure each of these on its own, because they usually get different paint. Ceiling area is length times width, so the 12 by 14 foot bedroom is 168 square feet. Two coats is 336 square feet, a little under one gallon at 350 per gallon.

Trim and doors are small areas but slow, detailed work, and they typically take a shinier paint than the walls. Count the doors and measure the baseboard and door casing in feet, then ask at the store how far a quart goes. Our posts on painting baseboards and painting interior doors cover those jobs.

Odd walls and accent walls

Not every wall is a rectangle. A wall under a sloped ceiling or a stairwell wall is easier to measure if you split it into rectangles and triangles. A triangle's area is half its base times its height, and you add the pieces together.

An accent wall is the simplest case. One 14 foot wall at 8 feet high is 112 square feet, or 224 for two coats. That's about two thirds of a gallon, and more than a quart covers, so buy a gallon or three quarts. Our post on accent wall ideas has more on picking the wall and the color.

What throws the count off

• Ceiling height. A 9 foot ceiling instead of 8 adds about 12 percent to your wall area.
• A big color change. Going from dark to light usually means a primer or extra coats, as our guide to painting over dark paint explains.
• Closets, built-ins and stairwells. They're easy to forget and they all take paint.
• The roller tray, the roller and the brush. They hold a surprising amount of paint, so round up.

Buying a little extra on purpose

Leftover paint isn't wasted if you store it well. Benjamin Moore says opened paint generally keeps about two years, and it suggests labeling the can with the brand, color, product name and sheen, plus where you used it, so a touch-up matches. Our post on storing leftover paint covers the storage, and how long paint lasts covers when to let it go.

If you hire a painter

You don't have to run this math if someone else is doing the work. On interior painting jobs we price by the square footage of the space, with paint included, so the gallons are our problem, not yours. We use premium paint from Sherwin-Williams and Behr, and Behr is the more economical of the two. If you'd rather use a brand you already like, tell us, and your written quote is built around the paint you pick.

George paints homes and businesses in Madison and the towns around it. It starts with a phone call, and he visits if he needs to see the space. You get a written quote with a start date, floors and furniture are covered before any paint comes out, small holes are repaired, and most rooms take one to two days.