Paint is the rare DIY purchase where guessing wrong is annoying in both directions: a fifth can from a different dye lot, or a gallon you'll dust in the garage for ten years. The math removes the guess.
The standard math
Wall area = perimeter × height. For the classic 14×12 ft room with 8-ft ceilings: 2 × (14 + 12) × 8 = 416 sq ft, minus ~70 sq ft of door and window = ~346 sq ft. Divide by the realistic coverage of 350 sq ft per gallon: almost exactly one gallon per coat. Two coats: two gallons. That's the whole arithmetic — the paint calculator does it for any room, and rounds up because partial cans and dye lots don't mix.
Why 350, when the can says 400?
Can labels quote ideal coverage: smooth, primed, perfectly thin application. Real walls are textured, porous, and get applied by humans with rollers that load unevenly. Professional estimators and most paint makers' own customer guidance converge on 300–350 for planning. Use 350 for interior repaints; mentally discount to 250–300 for raw drywall, brick, or heavy texture.
When you need more than the math says
- Dramatic color changes (white over navy, red over anything): a tinted primer under two coats beats three or four topcoats on cost and finish.
- Deep saturated colors: reds and some blues have notoriously weak hide — budget the primer without regret.
- Spray application: expect 20–30% overspray loss versus rolling.
- Bathrooms and kitchens: the square footage is small; moisture-rated paint often only comes in quarts anyway.
Two coats, always? (Mostly, yes.)
One coat of modern paint over a same-family color can look fine in daylight and patchy under evening lamps. Two coats is the standard because it delivers even color and scrubbability — the second coat is cheap insurance on the room's whole look. The one-coat exception: same-color refresh where you're really just renewing, not changing.
Buying checklist
- Run the calculator, round up to the next can size.
- Buy all cans at once — same brand line, same sheen, ideally same batch (check the lot code).
- Box (mix) cans together in a 5-gal bucket for big rooms to eliminate lot variation entirely.
- Add a quart of ceiling paint and trim enamel if those are in scope — they're cheap compared to a second trip.