How Much Paint Do I Need for a Room? Gallon Math Explained

A 12x12 bedroom with 8-foot ceilings needs about 2 gallons for two coats on the walls, and about 1 gallon if you're refreshing a similar color with a single coat. Your room will land somewhere near that, depending on its size, openings, and surface.

How Do You Calculate Gallons of Paint for a Room?

Gallons needed equal the paintable wall area, times the number of coats, divided by the square feet one gallon covers. Most interior wall paint lists roughly 350 to 400 square feet per gallon, and that figure assumes a smooth, previously painted surface.

We'd plan on 350. Check the paint coverage per gallon printed on your can, since it varies by brand and product, but the lower number keeps you from running short on the last wall.

The formula in one line: area x coats / 350 = gallons.

How Do You Measure Wall Area for Paint?

Measure the room's perimeter, then multiply by the floor-to-ceiling height. Run a tape along the floor at each wall, add the lengths, and multiply.

Treat bump-outs and alcoves as their own rectangles and add them in. A gable or vaulted wall is a rectangle plus a triangle, and the triangle is half the base times the height. The ceiling is separate: length times width. Trim, doors, and baseboards are separate again, and a quart often covers them.

Should You Subtract Doors and Windows?

Only when they're big. As a rule of thumb, subtract openings when together they take up more than about 10 percent of your wall area. A standard 3x7 door is about 21 square feet and a typical 3x5 window is about 15. Subtracting a couple of small ones shifts your total by a few percent, and that sliver is better left as cushion for cutting in, a thirsty paint roller, and touch-ups.

Large openings are different. A sliding glass door is around 40 square feet, and a picture window, a wall of built-ins, or a big brick fireplace removes a meaningful chunk. Ignore those and you may buy a gallon you never open.

Do You Need Two Coats of Paint?

Usually, yes, so double your wall area before dividing. One coat is realistic only when you're repainting the same or a very similar color over a clean, sound finish.

Covering a dark or saturated color with a lighter one is where estimates fall apart. Red, deep green, and navy often show through two coats of pale paint. Apply a tinted primer or stain-blocking primer first for an even base, then still plan on two finish coats. For that job, run the math for three coats.

Does Wall Texture Change How Much Paint You Need?

Yes, rough or porous walls use more paint than the label suggests. Orange peel and knockdown textures have more surface area than flat drywall, and stucco, brick, and raw wood have far more. New drywall and fresh plaster soak up paint too.

A common allowance is 10 to 20 percent extra for light texture. Heavy texture and rough masonry can cut coverage well below the label figure, so tell the paint counter what you're covering. Bare drywall needs a coat of drywall primer first, because finish paint on raw board is wasteful and tends to dry patchy.

Worked Example: How Much Paint for a 12x12 Bedroom?

A 12x12 bedroom with 8-foot ceilings has 384 square feet of gross wall area and needs just under 2 gallons for two coats once you subtract the openings. This example assumes one door and two windows, so you can compare it against your own room.

Those openings are about 13 percent of the wall area, so subtracting them made sense. Skip the subtraction and you'd get 2.2 gallons, which nudges you toward a third gallon. A single refresh coat comes to 333 / 350, or 0.95 gallon, so one gallon does it.

We'd pick 2 gallons plus a quart of paint from the same tint batch. The quart covers the thin margin and doubles as touch-up paint. The ceiling is 144 square feet, so one gallon of ceiling paint covers two coats with some left over.

What Size Cans Should You Buy, and How Do You Store Leftovers?

Buy the combination of cans that lands just past your calculated number, not the biggest can on the shelf. A quart covers roughly 85 to 100 square feet. Typical prices run $15 to $30 per quart, $35 to $80 per gallon for mid-range to premium interior paint, and $150 to $300 for a 5-gallon pail, depending on brand, sheen, and tint. If you need 2.2 gallons, two gallons plus a quart usually costs less than a third gallon.

Keep about a quart for touch-ups. Transfer it to a smaller airtight jar so there's little air inside, wipe the rim, and tap the lid down firmly. Label it with the room, color name, sheen, and date. Store it where it won't freeze or bake, ideally somewhere moderate. Latex paint that smells sour or has dried lumps is done.

For the rest, let small amounts of latex paint dry out completely before trashing them. Take oil-based paint and large volumes to a household hazardous waste drop-off. Rules vary by city and county, so check yours.

Your next step: measure your room tonight, run the formula with 350 square feet per gallon, and write the result on a note you can carry to the paint counter along with the surface type and the old color.