The short version: a single US gallon of interior paint covers roughly 350 square feet in one coat. Most walls need two coats, so a standard 12 × 12 room with one door and one window works out to about 2 gallons.
How to measure a room for paint
- Measure the perimeter. Add up the length of all the walls.
- Multiply by the wall height to get your total wall area in square feet.
- Subtract the openings. A standard door is about 21 sq ft and an average window about 15 sq ft.
- Add the ceiling only if you're painting it — that's simply length times width.
- Multiply by your number of coats, then divide by the paint's coverage rate.
Round up to the next full can. Running out two-thirds of the way through a wall means a colour-matching trip back to the store and a visible line where the coat dried unevenly.
Frequently asked questions
How much paint do I need for a 12 × 12 room?
For a standard 12 × 12 room with 8 ft walls, one door and one window, you need about 2 gallons for two coats of wall paint. Add roughly half a gallon more if you're also painting the ceiling.
Does one gallon of paint really cover 350 square feet?
On smooth, sealed walls, yes — roughly 350 sq ft per coat. On textured, porous or unprimed surfaces, expect closer to 250–300 sq ft, so plan for a little extra.