Guide
Buying paint is a balancing act: too little means a mid-project store run and the risk of a slightly different mixing lot, while too much leaves you with cans crowding the garage. The good news is that paint quantity is pure arithmetic. Once you know the coverage rule and a couple of adjustments, you can nail it every time.
One gallon of interior paint covers roughly 350 to 400 square feet in a single coat on a smooth, primed surface. That's the anchor for every paint calculation. Rough, porous, or unprimed surfaces soak up more and cover less, so lean toward the lower end (350) when in doubt.
For each wall, multiply width by height, then add the walls together. A 12 ft × 12 ft room with 8 ft ceilings has four walls totaling about 384 sq ft of surface (perimeter of 48 ft × 8 ft height).
You don't paint the glass or the door slab, so subtract them. Use handy averages: about 20 sq ft per door and 15 sq ft per window. One door and two windows would trim roughly 50 sq ft off the example above, leaving about 334 sq ft to paint.
Divide your paintable area by 350–400 to get gallons per coat, then multiply by the number of coats. Most color changes and fresh walls need two coats. Our example: 334 sq ft ÷ 350 ≈ 1 gallon per coat, so about 2 gallons for two coats. Always round up to whole cans.
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Once the gallon count is set, the finish comes down to the applicators. A good paint roller, tray, and brush kit is what turns that estimate into an even wall — bargain covers shed lint and leave streaks that can cost you an extra coat.
| Step | Example |
|---|---|
| Wall area | 384 sq ft |
| Minus 1 door + 2 windows | −50 sq ft → 334 sq ft |
| Gallons per coat (÷350) | ≈ 1 gallon |
| Two coats | 2 gallons |
Bare drywall, patched spots, or a dramatic color change usually call for primer, which follows the same coverage math. Ceilings are calculated separately as length × width. Trim and doors use small quantities but shouldn't be skipped in your budget.
That 350-to-400 figure isn't a fixed law — it's the number paint makers publish for a smooth, sealed wall getting one coat. Sherwin-Williams notes on its own paint calculator guidance that a gallon covers roughly 350–400 square feet, a quart about 100 square feet, and a gallon of primer only 200–300 square feet, because primer is formulated to soak in and grip rather than lay on top. Three things pull you toward the low end of that range — or below it:
Matching your purchase to the job means knowing what each can actually does. These figures track the ranges Sherwin-Williams publishes for a single coat on a primed, smooth surface:
| Container | One-coat coverage | Best for |
|---|---|---|
| Quart | ~100 sq ft | Trim, doors, touch-ups, one small accent wall |
| Gallon | 350–400 sq ft | One average room, single coat |
| 5-gallon bucket | ~1,750–2,000 sq ft | Whole floors, open-plan rooms, exteriors |
A 5-gallon bucket is also the easiest way to guarantee color consistency across a big job, since every wall is drawing from one continuous mix.
If your house was built before 1978, the existing paint you sand, scrape, or strip may contain lead. The U.S. EPA's Renovation, Repair and Painting (RRP) program requires paid contractors working in pre-1978 homes to be certified and to follow lead-safe work practices whenever a job disturbs more than six square feet of interior paint. The rule generally doesn't bind a homeowner working on their own home, but the health risk is identical — so the EPA urges the same precautions: contain the dust, mist surfaces before sanding, keep children and pregnant women away from the work area, and clean up thoroughly. When in doubt on old paint, an inexpensive lead paint test kit settles the question before you scrape.
The wall total is only part of the shopping list. Ceilings are their own calculation — length × width — and because they're often a flat white, they may take a different product than the walls. Trim, baseboards, and window casings sip paint rather than gulp it, but a whole house of trim can still add up to a gallon or two of a durable semi-gloss or enamel. A rough planning rule: budget a quart of trim paint per average room, and confirm with the coverage on the can. Doors are small — roughly 20 sq ft a side — yet a six-panel door with all its edges and profiles eats more than the flat area suggests, so don't cut it too close.
How much paint for a 12 × 12 room? About two gallons for two coats, following the worked example above — one gallon per coat once doors and windows are subtracted.
Will one gallon do a small bedroom? For a single coat on an average bedroom, usually yes; for two coats, plan on two gallons.
How long does leftover paint keep? Sealed tightly and stored somewhere that never freezes, latex paint often stays usable for a few years — handy for touch-ups and a strong argument for buying that extra quart.
To skip the pencil work, the paint calculator lets you enter room dimensions, doors, windows, and coats, then returns the exact number of gallons — coverage math and rounding handled for you.