12 × 12 is the classic bedroom size. Here is the paint for the walls at the normal two coats, and what changes if you add the ceiling.
For a 12 × 12 room, 8 ft walls, 2 coats: buy 2 gallons for the walls. Painting the ceiling too: 3 gallons in total.
Step 1: Wall area the four walls make a band around the room: 2 × (12 + 12) × 8 = 384 sq ft.
Step 2: Take off doors and windows a normal door counts 20 sq ft, a window 15. 384 − 20 − 15 = 349 sq ft to paint.
Step 3: Two coats 349 × 2 = 698 sq ft of painting. One gallon covers about 350 sq ft, so 698 ÷ 350 = 1.99 gallons exact.
Step 4: Round up shops sell whole gallons, so buy 2 gallons. With the ceiling (+144 sq ft) the same math gives 2.82, buy 3.
Other room sizes, 8 ft walls, 1 door, 1 window, 2 coats, walls only:
| Room size | Paintable area | Gallons to buy |
|---|---|---|
| 10 × 10 ft | 285 sq ft | 2 gallons |
| 12 × 10 ft | 317 sq ft | 2 gallons |
| 12 × 12 ft | 349 sq ft | 2 gallons |
| 15 × 12 ft | 397 sq ft | 3 gallons |
| 20 × 15 ft | 525 sq ft | 3 gallons |
For a color change, yes, one coat almost always shows the old color through. Repainting the same color in good condition can be one coat: for this room that is 349 ÷ 350 ≈ 0.997 of a gallon, so 1 gallon is enough.
No, primer is separate. You need it on new drywall, stains, or a big color change, about one gallon for this room's walls.
Trim usually takes a different paint (semi-gloss) and a small amount, one quart often covers a room's trim. This page counts wall paint only.