Paint Calculator - How Much Paint Do I Need for a Room?

How to measure a room for paint

You need three numbers: the length and width of the floor and the height of the walls. Measure wall to wall at the baseboard, round up to the nearest half foot, and measure height from the floor to the ceiling, not to the top of the crown molding. A laser measure makes this a two-minute job, but a 25-foot tape is fine.

Then count doors and windows. The calculator subtracts 21 sq ft per door (a standard 3 × 7 ft interior door) and 15 sq ft per window (roughly 3 × 5 ft). A sliding patio door is closer to 40 sq ft, so count it as two doors. A wall of big picture windows? Count it as three or four windows. Closets you plan to paint count as extra wall area, so add their width to the room length.

The formula

Wall area = 2 × (length + width) × height. That is the perimeter of the room times the wall height. Subtract the doors and windows, multiply by the number of coats, and divide by the coverage on the can. Most interior paints list 350 to 400 sq ft per gallon; Sherwin-Williams and Benjamin Moore both quote that range. We default to 350 because real walls soak up more than a lab panel does.

Texture matters more than most calculators admit. Orange peel or knockdown drywall has more surface than it looks, so the tool trims coverage by 10%. Brick, block, and stucco drink paint, so it drops 25%. If you are covering new drywall or bare wood, tick the primer box: primer covers about 200 to 300 sq ft per gallon and you only need one coat.

Worked example: a 14 × 16 living room

Say the room is 14 by 16 feet with 9-foot ceilings, two doors, and three windows. The perimeter is 60 feet, so the walls are 60 × 9 = 540 sq ft. Take off 42 sq ft for the doors and 45 for the windows and you have 453 sq ft to paint.

Two coats means 906 sq ft of coverage. At 350 sq ft per gallon that is 2.59 gallons. In cans, that rounds up to 2 gallons plus 3 quarts, but three quarts cost more than a fourth gallon, so the calculator tells you to buy 3 gallons. The leftover is your touch-up stash for the next scuffed corner.

Painting the ceiling too? It is 224 sq ft, so two coats is 1.28 gallons, which means 1 gallon plus 2 quarts of flat ceiling white. Most people use one gallon and stretch it; do not. A thin second coat on a ceiling shows every roller line when the light hits it.

How many coats do you really need?

One coat works only when you are refreshing the same color on walls in good shape. Two coats is the default for almost everything else, including paint-and-primer products; the label claim assumes a similar color underneath. Going from dark to light, or painting a saturated red, yellow, or orange, often takes three coats or a tinted primer plus two. Gray-tinted primer under deep colors saves a coat and usually costs less than a gallon of the premium topcoat.

Common mistakes

Buying for one coat. The coverage number on the can is per coat. Double it.

Forgetting texture and porosity. Fresh drywall mud and patched areas soak paint unevenly. Prime them, or the finish will flash (show dull patches) no matter how many topcoats you add.

Mixing batches mid-wall. Custom tints vary slightly between cans. If you need three or more gallons, pour them all into a 5-gallon bucket and stir. Painters call this boxing, and it stops a visible line where one can ended and the next began.

Rounding down to save money. A trip back to the store costs more than a quart, and a second batch may not match exactly.

When to hire a pro

A standard bedroom is a weekend DIY job. Hire out when ceilings are over 10 feet, when there is lead paint risk (homes built before 1978 need lead-safe practices when you sand or scrape), when walls need heavy patching or skim coating, or when you are painting stairwells that need scaffolding or a ladder plank. Pros also cut a clean line along ceilings far faster than most homeowners, which is most of what separates a good paint job from a DIY-looking one.