Short answer: for a 12×12 room with 8 ft walls, you need about 2 gallons of paint for two coats on the walls. Buy 2 gallons plus 1 quart (2.25 gallons total) so you are not caught short by texture or touch-ups. Painting the ceiling too? Budget roughly 3 gallons in total.
The breakdown
Here is exactly where the number comes from. Four walls at 12 ft wide and 8 ft high give 384 sq ft of wall. Subtract one standard door (21 sq ft) and two average windows (15 sq ft each) and you have 333 sq ft of paintable surface.
| Coats | Paint to buy |
|---|---|
| 1 coat | About 0.95 gallons — buy 1 gallon |
| 2 coats (recommended) | About 1.9 gallons — buy 2 gallons + 1 quart |
| 3 coats | About 2.85 gallons — buy 3 gallons |
| Ceiling added (2 coats) | Add about 0.8 gallons — buy 3 gallons total |
This assumes one door and two windows. With one window, the paintable area is 348 sq ft and two coats need 1.99 gallons — buy 2 gallons. See also our bedroom paint guide.
Do I need primer as well?
On new drywall, bare wood, or when covering a much darker color, yes — one coat of primer first. For this room that is about 1.1 gallons of primer, so buy 1 gallon plus a quart. Primer is cheaper than paint and makes the topcoat cover in two coats instead of three.
Tips before you buy
Buy all your paint in one trip from the same batch — mixed colors vary slightly between batches. Keep the leftover in a sealed, labeled jar for touch-ups; a quart saved now beats a mismatched wall later. And if your walls are heavily textured, knock the coverage down to 250–300 sq ft per gallon in your estimate.
Different room size? Run your own numbers in our free paint calculator — it handles doors, windows, ceilings and primer for any room.