A 12 × 12 ft room with 8 ft walls, one door and one window needs about 2 gallons of paint for two coats, so buy 2 gal. That assumes a typical 350 sq ft per gallon coverage. The ceiling, if you paint it, takes another 1 gal of ceiling paint.
The math for a 12 × 12 room
- Wall area: perimeter × height = (12 + 12 + 12 + 12) × 8 = 384 sq ft.
- Openings: a standard door is about 21 sq ft (3 × 7 ft) and an average window about 15 sq ft. 384 − 21 − 15 = 348 sq ft to paint.
- Two coats: 348 × 2 = 696 sq ft of coverage needed.
- Gallons: 696 ÷ 350 = 1.99 gallons.
Notice how close that is to exactly 2 gallons. Leave out the door and window and it becomes 2.19 gallons, which tips the purchase to 2 gal + 1 qt. In practice, two gallons does a 12 × 12 room with normal openings, and an extra quart covers touch-ups. With 9 ft walls it’s 2.26 gallons: buy 2 gal + 2 qt.
One coat is 0.99 gallons, but a single coat rarely covers well unless you’re repainting the same color. Plan on two.
Paint coverage chart by room size
Two coats, 350 sq ft per gallon, one door and one window per room:
| Room | Wall area, 8 ft walls (sq ft) | Gallons, 8 ft walls | Buy, 8 ft walls | Buy, 9 ft walls | Ceiling, 2 coats |
|---|---|---|---|---|---|
| 10 × 10 ft | 284 | 1.62 | 2 gal | 2 gal | 1 gal |
| 10 × 12 ft | 316 | 1.81 | 2 gal | 2 gal + 1 qt | 1 gal |
| 12 × 12 ft | 348 | 1.99 | 2 gal | 2 gal + 2 qt | 1 gal |
| 12 × 14 ft | 380 | 2.17 | 2 gal + 1 qt | 2 gal + 2 qt | 1 gal |
| 14 × 14 ft | 412 | 2.35 | 2 gal + 2 qt | 3 gal | 1 gal + 1 qt |
| 12 × 16 ft | 412 | 2.35 | 2 gal + 2 qt | 3 gal | 1 gal + 1 qt |
| 16 × 16 ft | 476 | 2.72 | 3 gal | 3 gal + 1 qt | 1 gal + 2 qt |
| 16 × 20 ft | 540 | 3.09 | 3 gal + 1 qt | 3 gal + 2 qt | 2 gal |
| 20 × 20 ft | 604 | 3.45 | 3 gal + 2 qt | 4 gal | 2 gal + 2 qt |
For your own room, the paint calculator lets you set the number of doors and windows, the coverage on your can and whether to include the ceiling and primer.
Coverage changes the answer more than you’d think
Labels quote anywhere from about 250 to 400 sq ft per gallon. Same 12 × 12 room, two coats:
| Coverage | Gallons | Buy |
|---|---|---|
| 250 sq ft/gal | 2.78 | 3 gal |
| 300 sq ft/gal | 2.32 | 2 gal + 2 qt |
| 350 sq ft/gal | 1.99 | 2 gal |
| 400 sq ft/gal | 1.74 | 2 gal |
Read the coverage on the can you’re buying, not a generic figure. Thick one-coat paints often claim lower square feet per gallon because they go on heavier.
Ceilings are a separate purchase
Ceiling paint is usually flat and formulated to resist spatter, so it’s a different product from wall paint. A 12 × 12 ceiling is 144 sq ft: two coats at 350 sq ft per gallon is 0.82 gallons, so one gallon is enough.
Trim and doors
Baseboard, door casing and the door itself take very little paint by area. For one room, a quart of trim enamel is almost always enough, even for two coats.
Tips that save a trip back
- Buy it all at once. Custom-tinted paint from two batches can differ slightly. If you need to mix batches, box them: pour all the cans into a bucket and stir.
- Keep a little. Label a small jar with the room and color for touch-ups.
- Prime when the color changes a lot, or over patched drywall. Primer makes the topcoat cover in fewer coats.
- Count closets. A reach-in closet adds about 60 to 80 sq ft of wall if you paint inside it.