The short answer
A 12 × 12 ft room with 8 ft walls, one door and two windows has 334 sq ft of paintable wall. Two coats at 350 sq ft per gallon need 1.91 gallons, so buy 2 gallons, about $90. Add the ceiling and the list grows by 1 gallon.
Worked example
| Step | Math | Result |
|---|---|---|
| Gross wall area | 2 × (12 + 12) × 8 | 384 sq ft |
| Openings | 1 door × 20 + 2 windows × 15 | − 50 sq ft |
| Paintable area | 384 − 50 | 334 sq ft |
| Gallons (2 coats) | 334 × 2 ÷ 350 | 1.91 gal |
| Buy this | cheapest cans that cover 1.91 gal | 2 gallons = $90 |
Why a can mix, not just “round up”
Rounding up is fine for small jobs. For bigger ones, the cheapest way to buy changes: at the default prices, 2.1 gallons is 2 gallons + 1 quart ($110), not 3 gallons ($135), and once you need more than 4 gallons, a 5-gallon bucket ($200) costs the same as four gallons plus a quart and gives you a full 5 gallons. We always show the round-up price too, so you see both.
What counts as trim
The trim option adds baseboard (the room perimeter minus 3 ft per door), door casing (17 ft per door) and window casing (14 ft per window), about 4 inches wide. It is a rough estimate, usually a quart or so per room.
Coverage: most cans say 300–400 sq ft per gallon, and PaintCare’s Buy Right guide suggests 350 for a rough estimate. Rough textures can need up to 50 percent more paint (PaintCare).
FAQ
How do I calculate how much paint I need?
Wall area = room perimeter × wall height, minus doors and windows. Multiply by the number of coats and divide by the coverage on the can (350 sq ft per gallon if unknown). A 12 × 12 ft room with 8 ft walls, one door and two windows needs 1.91 gallons for two coats.
How many gallons of paint for a 12x12 room?
About 2 gallons for walls with two coats (1.91 gallons exact). Add 1 more gallon if you also paint the ceiling.
Do I subtract doors and windows?
Yes. We subtract 20 sq ft per door and 15 per window by default. You can change those sizes under Prices, waste and more.
Should I buy extra paint?
A small spare is wise for touch-ups, and the leftover from a mix is shown. You can also add 5 or 10 percent for waste under Prices, waste and more.
Are the prices real?
They are typical 2026 US mid-grade interior prices ($45 gallon, $20 quart, $200 for 5 gallons). Edit them to match your store.