Paint calculator

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

The openings have to come off. 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%. An estimate built from length, width and height alone quietly adds a 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.

Where this estimate stops

Openings come off at fixed sizes. Doors and windows are counts, not measurements: each one takes off a 3 × 7 door or a 3 × 5 window and there is no field for the real thing. A patio slider, a bay or a wall of glass has to be measured and subtracted by hand. Enter "one window" for a 40 sq ft picture window and you have left most of it on the estimate.

The subtraction has a floor at zero. The openings come off the wall area and the result is clamped at zero, so if the doors and windows you enter add up to more than the walls do, the page returns 0 sq ft and 0 gallons. That is the arithmetic declining to go negative, not an answer — it means the counts do not describe the room. The same thing happens for real on a wall that is mostly glass: work the paintable area out by hand and enter it as a smaller room.

A zero in the length or the width returns nothing at all, for the same reason — half a room is not a room, and the two walls the perimeter would leave standing on it are not walls you can paint. The taken off for doors and windows line keeps reporting through that, because it is a property of the counts you entered rather than of the room, and it is how you check the openings before you measure.

On the ceiling, the opening counts stop doing anything. Switch the surface to the ceiling and the page paints length × width and nothing else — the door and window counts and the ceiling height are all ignored, which is right for a flat ceiling and wrong for a skylight or an open stairwell. Take those off yourself.

The geometry is a box. Wall area is perimeter times height, so an L-shaped room, a stairwell, the gable end over a vaulted ceiling and a soffit dropped above the cabinets are all outside it. Split an awkward room into rectangles and run the page more than once.

The surface multiplier is our reading of a published range, not a published multiplier. Behr states 250–400 sq ft per gallon for Premium Plus Interior and puts the spread down to "surface texture, porosity and application method". The extra we add to the first coat on bare or patched board is what it takes to land at the bottom of that range starting from a working 375. Behr does not print a per-substrate factor and neither does anyone else, so treat the choice as a dial you are allowed to move rather than a citation.

Two things the coverage figure quietly assumes: that the wall is flat, when a knockdown or popcorn texture has more surface on it than its floor plan suggests; and that you are rolling it. Spraying puts paint into the air and onto the drop cloths at a rate the 5% allowance here does not try to carry.