Paint calculator
Figure out exactly how many gallons to buy for a room — walls, doors, windows, and number of coats all factored in.
Room details
How this is calculated
Wall area is the room's perimeter times the ceiling height: 2 × (length + width) × height. Each door removes about 20 sq ft and each window about 15 sq ft, since paint doesn't go on glass or door slabs. That paintable area is multiplied by your number of coats, divided by the paint's coverage rate (350 sq ft/gallon is standard for most interior paints — check your can if you know the brand), then increased by 10% to cover trim work, touch-ups, and a second coat that never spreads quite as far as the first.
How much paint for a 12×12 room?
At 8 ft ceilings with one door and two windows, two coats needs roughly 2.5 gallons — round up to 3 gallons, or buy one 5-gallon bucket if you're also doing an adjoining room.
Do I need to paint the ceiling too?
This calculator covers walls only. Ceilings usually take flat ceiling paint rather than wall paint — run the room's length × width through the same coverage-rate math separately.
What if my paint's coverage rate is different?
Check the label on the can — some premium and textured paints cover closer to 250–300 sq ft/gallon. Adjust the coverage field above and the result updates automatically.
Once you know how much to buy
A few things that make the job go smoother — these are affiliate links, so we may earn a small commission if you buy through them.
Interior wall paint, 1 gal
Standard coverage ~350 sq ft/gal, matches the default in this calculator.
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