Paint math is simple; the part people get wrong is the openings and the coats. A 12 by 14 room with an eight-foot ceiling has 416 square feet of wall gross, but with a door and two windows it is closer to 371 — and at two coats that is still two gallons, not one.
Coverage is per coat
The 350 to 400 square feet on the can is one coat on a smooth, sealed, similarly coloured surface. Two coats is the normal expectation for a decent finish, so double it.
New drywall, bare wood, and any dramatic colour change need primer first, which is a third pass. Going from dark to light can need two coats of primer.
What changes coverage
Texture eats paint. A knockdown or orange-peel wall can pull coverage down to 300 square feet per gallon; heavy texture lower still.
Colour matters too. Deep reds, yellows and oranges have weak pigment opacity and often need three coats regardless of how carefully the first two went on. A tinted primer close to the final colour fixes this more cheaply than a third coat.
Subtracting openings
A standard interior door is about 21 square feet and an average window about 12 to 15. This calculator uses those figures.
Do not subtract anything smaller than that. Trim, outlets and switch plates are not worth deducting, and the rounding works in your favour on the cut-in.
Ceilings and trim are separate
Ceiling paint is a different product and a separate calculation. So is trim, which is bought by the quart more often than the gallon.
A rough rule for trim: one quart covers about 100 linear feet of standard baseboard at two coats.
Sheen, and where each one belongs
Flat hides surface imperfections and cannot be scrubbed. Gloss scrubs well and shows every flaw. Everything else sits between the two.
The usual assignment is flat or matte on ceilings and low-traffic walls, eggshell or satin on most walls, satin or semi-gloss in kitchens and bathrooms, and semi-gloss or gloss on trim and doors.
Sheen also affects coverage slightly — higher sheen products tend to spread a little further — but the difference is small next to the effect of surface texture.
Keeping the leftover usable
Paint stores badly in a partly full can. Air in the can skins the surface and eventually ruins it, and the rim collects dried paint that stops the lid sealing.
Wipe the rim before closing, seal the lid properly, and store the can upside down if it is well sealed — the paint itself makes the seal.
Write the room and the date on the lid. A year later the colour name on the label means nothing, and matching from a dried sample is far harder than reading a note.