How much paint for a 12x12 ceiling?

How the number is worked out

Wall area is the perimeter times the ceiling height: a 12 × 12 room with an 8 ft ceiling is 2 × (12 + 12) × 8 = 384 sq ft. Take out one door (21 sq ft) and one window (15 sq ft) and you are painting 348 sq ft, not 384.

Then each coat is divided by the coverage printed on the can. Sherwin-Williams publishes 350–400 sq ft per gallon on the data sheets for SuperPaint Interior Satin, ProMar 200 and CHB Interior Latex Flat, measured at 4 mils wet. Behr publishes 250–400 sq ft per gallon for Premium Plus Interior, and says outright that the figure depends on "surface texture, porosity and application method". Both statements are on the same page as each other: 375 is a fair working number on a wall that is already painted and smooth.

The two mistakes that cost you a trip to the store

Nobody deducts the openings. A 12 × 12 room loses about 10% of its wall area to one door and one window. On a 12 × 20 living room with a patio door and three windows it is closer to 20%. Every calculator that asks only for length, width and height is quietly selling you an extra gallon.

The first coat is not the same as the second. Bare drywall, joint-compound patches and any texture are porous — the first coat sinks in and you get nowhere near the number on the can. That is the whole reason Behr's published range runs down to 250 sq ft/gal. Doubling the area and dividing by 375 under-buys the first coat and is exactly why people run out with one wall to go. This calculator prices the coats separately.

The clean fix is to prime bare board first. A gallon of primer is cheaper than a gallon of colour, it kills the porosity, and the two colour coats then behave like the can says.

Buying it

Paint comes in gallons and quarts, and a quart is roughly a quarter of the price of a gallon. If the answer lands at 1.2 gallons, buy one gallon and one quart rather than two gallons — it is a real saving on a small room, and you still have touch-up paint left.

Above about three gallons the maths flips: five-gallon buckets are cheaper per gallon, and mixing everything from one bucket removes any risk of a batch mismatch showing up on the wall. If you do buy two separately-tinted gallons, box them — pour both into one bucket and stir — before you start.

Trim and ceiling are separate orders. Ceiling paint is a dead-flat product that is deliberately thick, so it covers less per gallon than wall paint; drop the coverage figure to around 350 when you switch this calculator to the ceiling.