The spread rate is the whole calculation, and it is a guess
Everything downstream of the area is arithmetic. Area divided by spread rate, times coats. The area you can measure. The spread rate you cannot, and it is the number that decides whether you buy two gallons or three.
The figure on the can, usually 350 to 400 square feet per gallon, is measured under favourable conditions: primed and sealed drywall, a light colour going over a similar light colour, applied at the film thickness the manufacturer intends. Real rooms fall short of that in ordinary ways. New drywall drinks paint until it is sealed. Skim-coated patches drink more than the wall around them, which is why they flash through the finish coat. A heavy nap roller lays down more paint per pass than a short nap. Cutting in with a brush uses more per square foot than rolling. Spraying wastes a percentage into the air even with good technique.
| Situation | Realistic spread | Why |
|---|---|---|
| Primed, sealed drywall, similar colour | 350-400 sq ft/gal | The label case |
| Previously painted, mild colour change | 300-350 sq ft/gal | Normal repaint |
| New or patched drywall, unprimed | 200-275 sq ft/gal | Substrate absorbs the first coat |
| Knockdown or orange peel texture | 250-300 sq ft/gal | More surface than the flat measurement suggests |
| Bare masonry, block, stucco | 100-200 sq ft/gal | Deep porosity, the first coat mostly disappears |
If you are unsure, use the lower end. Being half a gallon over costs you a can you keep for touch-up. Being half a gallon short costs a second trip and a batch that may not match.
Deducting doors and windows, and when not to
The defaults here take 21 square feet per door, which is a standard 3 by 7 foot opening, and 15 square feet per window, roughly 3 by 5. Those are opening sizes, not trim sizes. If you are painting the trim in the same colour as the wall, do not deduct at all, because the trim gives back the area the opening took away and then some.
For a room with one door and two average windows the deduction is 51 square feet, about a seventh of a gallon at 350. It matters when it tips you across a can boundary and is otherwise noise. Large glazing changes that: a wall that is mostly window can halve the paint for that wall, and it is worth measuring properly rather than counting openings.
Quarts, gallons and pails
Paint is sold in three sizes and the arithmetic does not respect them. A calculation that says 2.1 gallons means you buy two gallons and a quart, or three gallons, or a 5-gallon pail if you have other rooms in the same colour. The pail is normally the cheapest per gallon by a clear margin, and it has a second advantage that nobody mentions on the shelf: it is one batch. Tinted paint varies slightly can to can, enough to see on a long wall in raking light. Pros box-mix multiple cans into a bucket before starting for exactly this reason. If your job needs four or more gallons, a pail solves both problems at once.
Quarts are the opposite trade. They cost far more per gallon than any other size and exist for touch-up and small work. Buying two quarts to top up a job is usually worse value than buying the extra gallon and keeping what is left.
Primer is a volume question and a cost question at the same time
The calculator treats primer as a separate coat at a marginally higher spread rate, because primer is thinner-bodied than finish paint. Whether you need it is not a volume question. Prime when the substrate is new or repaired, when you are going light over dark, when the existing finish is glossy, when there is anything to block such as smoke, tannin or water stain, or when you are painting bare masonry or wood. Skip it when you are putting a similar colour over a sound, matte, previously painted wall.
The economic case usually favours priming even when coverage would eventually get there without it. Primer costs less per gallon than finish paint and seals a porous surface so the finish coats spread at the rate they are supposed to. Two coats of expensive colour over unsealed drywall often costs more than one coat of primer plus two thinner coats of colour, and looks worse.
Questions people ask
How much paint do I need for a 12 by 12 room?
With eight-foot ceilings a 12 by 12 room has 384 square feet of gross wall. Take out one door and two windows and you have about 333 square feet. Two coats at 350 square feet per gallon is 1.9 gallons, so two gallons buys it with a little spare. Drop the spread rate to 300 for unprimed or patched walls and the same room needs 2.2 gallons, which means two gallons plus a quart. Add the ceiling and you are adding 144 square feet, another 0.4 to 0.5 gallons per coat — ceilings are normally done in one coat of flat ceiling paint rather than two.
Do I really need two coats?
Almost always, yes. One coat looks acceptable while wet and then dries into visible variation, especially on walls that catch side light. The exceptions are narrow: a near-identical colour over a sound sealed surface, or a high-hide paint over a properly tinted primer where the manufacturer specifically rates it for single coat. What actually reduces coats is priming and tinting the primer toward the finish colour. Going dark to light is the case where people try one coat, and it is the case where it never works — count on two coats of finish over a tinted primer, sometimes three.
Does the calculator handle trim, doors and cabinets?
No. It sizes wall and ceiling area, which is flat area you can measure. Trim, doors and cabinetry are surface-area problems of a different shape and are usually bought in quarts rather than gallons. As orientation, a quart of trim enamel typically does one average interior door with two coats, or roughly 100 linear feet of standard baseboard with two coats. Those figures move a lot with the profile of the moulding and how much of it you can spray.
Why does the store recommend more paint than this?
Sometimes because they are using a lower spread rate, which may well be the right call for your walls. Sometimes because they are rounding to the next whole gallon on each coat rather than on the total, which inflates the count. And sometimes because the incentive runs one way — nobody has ever been complained at for selling a customer an extra gallon. Ask what spread rate the recommendation assumes. If it is 250 to 300 for a repaint over sound painted drywall, that is conservative, and you can decide whether you want the margin.