How Much Paint Do I Need?
Paint coverage comes down to one number: your paintable wall area, in square feet, divided by how far a gallon stretches. A gallon of interior wall paint typically covers about 350 square feet in a single coat on primed, smooth drywall. Rough, textured, or previously unpainted surfaces drink more, so treat 350 as a friendly average, not a guarantee.
Getting the area right
Start with the four walls: add the room's length and width, double it, and multiply by ceiling height. That gives total wall area. Then subtract the openings you won't paint — a standard door is roughly 21 square feet and an average window about 15. The calculator does this for you, so you enter dimensions and door and window counts rather than doing the arithmetic. Most rooms need two coats, especially over a color change or fresh primer, and two coats simply doubles the area your paint has to cover.
Buy a little extra
Always round up to whole gallons and keep a bit in reserve. Touch-ups months later are far easier with leftover paint from the same can than with a fresh gallon that may have shifted slightly in tint. If you're between sizes, a gallon plus a quart often beats buying two gallons you'll mostly waste. And if you're painting a ceiling too, calculate it separately — ceiling paint and coverage differ from walls.
Frequently asked questions
- How many gallons for a 12x12 room?
- With 8-foot ceilings and two coats, a 12x12 room needs roughly 2 gallons after subtracting a door and windows.
- Does one gallon really cover 350 square feet?
- On smooth, primed drywall, yes. Textured or bare surfaces can drop that to 250-300, so measure generously.
- Should I count the ceiling?
- Only if you're painting it. Calculate ceilings separately since their area and paint type differ from walls.