Measure the room once. Get a separate, store-ready paint list for walls, ceiling, trim and primer.
wall area = 2 × (length + width) × height − doors − windows
gallons = area × coats ÷ coverage for your surfaceSherwin-Williams and Behr both quote 350–400 sq ft per gallon per coat on smooth primed drywall. Rougher and more porous surfaces take far more: smooth drywall or plaster 400, textured or previously unpainted drywall 350, wood siding or panelling 275, brick or block 200, stucco 175 sq ft per gallon. Picking the wrong one here is the single biggest source of a bad paint estimate.
Primer is calculated separately at 250 sq ft per gallon — it's thinner bodied than finish paint and covers noticeably less ground, which is why one gallon of primer does not stretch as far as one gallon of paint.
Ceilings are counted as length × width. Trim is estimated from the room perimeter, which covers baseboards and door casing for a typical room — if you have deep crown moulding, buy an extra quart.
Walls, ceiling and trim are listed separately because they take different paint. Ceiling paint is flat, trim is usually semi-gloss or gloss, and you cannot pour one into the other.
The ceiling defaults to one coat while the walls default to two. Ceiling paint is thick, flat and usually going over white, so a second coat is rarely needed. Change it if you're covering a stain, a bold colour, or a ceiling that has never been painted.
A 12 ft by 12 ft room with 8 ft walls has 384 sq ft of wall before openings. Take off one door and two windows and you are at 333 sq ft, which is about 1.7 gallons for two coats on smooth drywall — so two gallons. Add roughly two thirds of a gallon if you are painting the ceiling too.
350 to 400 sq ft per coat on smooth primed drywall, which is what Sherwin-Williams and Behr both quote. Rougher surfaces are dramatically worse: wood siding around 275, brick 200, and stucco only 175 sq ft per gallon. Choosing the wrong surface is the biggest single cause of a bad paint estimate.
Two, in almost every real situation. One coat only works when you are repainting the same colour on a sound, already-painted wall. Going lighter over a dark colour, covering patches, or using a strong colour with poor natural opacity — reds and yellows especially — can need three.
Primer covers about 250 sq ft per gallon, noticeably less than finish paint, because it is thinner bodied. It goes on once over everything you are painting. You need it on bare drywall, patched areas, bare wood, and when covering a dark colour with a much lighter one.
Yes, and this calculator does it for you at 21 sq ft per door and 15 sq ft per window — the standard sizes. On a small room those openings can be 15% of the wall area, which is easily a whole gallon on a large job. Unusually large picture windows or sliding doors will make the estimate run high.
Not well, which is why this calculator lists them separately. Ceiling paint is flat to hide imperfections and formulated not to drip. Trim paint is semi-gloss or gloss so it wipes clean and stands up to knocks. They are different products in different tins and you cannot substitute one for the other.
Trim here is estimated from the room perimeter, which covers baseboards and door casing on a typical room. That usually lands under a gallon for a single room. If you have deep crown moulding, panelling, or tall Victorian skirting, buy an extra quart.