Measure the room once. Get a separate, store-ready paint list for walls, ceiling, trim and primer.
A gallon covers about 350 to 400 square feet on smooth, previously painted drywall. That figure is on the can and it assumes near-ideal conditions, so treat it as a ceiling rather than a promise.
Texture is the biggest variable. Knockdown or orange peel texture has more surface area than it appears to, and light texture drops coverage to roughly 300 square feet per gallon. Heavy texture, popcorn ceilings and stucco can halve it. Bare drywall, new plaster and raw wood drink the first coat regardless of texture, which is what primer is for.
Colour change matters more than most people expect. Going from white to a mid tone is straightforward; going dark over light, or light over dark, is a three-coat job however good the paint is. Deep reds and saturated yellows are the worst offenders because the pigments are transparent — a red accent wall over white regularly takes three or four coats.
Application method changes the number too. Spraying puts more paint in the air and on the surface than rolling, and a sprayed job typically uses 20 to 30 percent more material even though it goes faster.
Two coats is the working assumption for almost every repaint, and it is not a manufacturer upsell. One coat over an existing colour looks acceptable when wet and shows lap marks, roller texture and patchy sheen once it dries and the light moves across the wall.
The second coat is also what delivers the durability the paint is rated for. Scrubbability, stain resistance and colour retention are all specified at the full film thickness, which one coat does not reach.
"One-coat" paints are thicker, higher-solids products that genuinely can cover in one pass — over a similar colour, on a smooth prepared surface, applied generously. Over a colour change or bare substrate they behave like any other paint.
Where one coat is genuinely enough: refreshing a wall with the exact same paint, and painting a ceiling that was recently done in the same white.
Primer is not a cheaper first coat. It is a different product that bonds to difficult surfaces and seals porous ones so the finish paint stays on the surface where it does its work.
You need it on bare drywall, new plaster, bare wood and any patched or skim-coated repair — otherwise the repaired spots absorb differently and flash through the finish as dull patches. You need it over glossy surfaces after sanding, and over any water, smoke or tannin stain, where a stain-blocking primer is the only thing that stops the mark bleeding through however many topcoats go on. Cedar and redwood bleed tannin indefinitely without one.
A drastic colour change is easier with a tinted grey primer than with extra topcoats, and it is cheaper — primer costs less per gallon than finish paint.
You can skip it when repainting a sound, previously painted wall in a similar colour with no repairs. Primer covers around 200 to 300 square feet per gallon, less than finish paint, because it is doing a different job.
Sheen decides how the wall survives daily life, and how much of the wall you see. The flatter the finish, the better it hides imperfections and the worse it takes cleaning.
Flat and matte hide roller marks, drywall seams and wavy walls, which makes them right for ceilings and for adult bedrooms and formal rooms. They mark when wiped.
Eggshell and satin are the general-purpose choices — enough sheen to wipe down, little enough to stay forgiving. Eggshell for living rooms and hallways, satin where hands and moisture reach.
Semi-gloss and gloss go on trim, doors, cabinets and bathrooms. They clean up well and take repeated wiping, and they show every flaw underneath, so surface preparation has to match.
A useful rule: the more a surface gets touched, the more sheen it needs; the more imperfect it is, the less it can take.
Paint is tinted in batches, and two gallons of the same colour mixed on different days can differ slightly. The fix is to buy all of it in one purchase, and for a large job to box it — pour every gallon into a single bucket, stir, and work from that.
Buying in gallons is also cheaper than buying in quarts once you need more than about two quarts, and a five gallon bucket is cheaper again per gallon if the job is big enough. Round up to the next whole container rather than trying to land exactly.
Keep the leftovers with the colour code and the store name written on the lid, not just the manufacturer name. Touch-up paint that matches is worth more than the few dollars the extra quart cost, and computer colour matching from a chip is never quite as close as the original formula.
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.