Common Mistakes When Painting a Room (and How Much to Buy)
Paint is cheap to buy twice and annoying to run out of halfway down a wall. A 12 × 12 ft room with 8 ft walls, one door and two windows needs about 1.9 gallons for two coats on the walls at 350 sq ft per gallon. The paint calculator does the same sum for your own room. These are the mistakes that most often change the answer.
Part of the project planner: Planning to paint a room or a house.
Mistake 1: buying for one coat
The can label usually covers one coat. A new color over an old one, or bare drywall, almost always needs two. Gallons for the walls of the example room:
| Coats | Gallons |
|---|---|
| 1 | 1.0 |
| 2 | 1.9 |
| 3 | 2.9 |
Mistake 2: leaving out (or double counting) doors and windows
Openings take paint off the total, but not by much. Ignoring them is a safe mistake that wastes a little, while counting every opening twice can leave you short.
| Openings | Gallons for walls |
|---|---|
| 0 doors, 0 windows | 2.2 |
| 1 door, 0 windows | 2.1 |
| 1 door, 2 windows | 1.9 |
| 2 doors, 4 windows | 1.6 |
Mistake 3: trusting the coverage on the can
A can says about 350 to 400 sq ft per gallon, but rough, dark or porous walls soak up more. Rate the surface honestly. The table shows the gallons for the example room at four coverage rates, without and with the ceiling:
| Coverage | Walls only | Walls and ceiling |
|---|---|---|
| 250 sq ft per gallon | 2.7 | 3.8 |
| 300 sq ft per gallon | 2.2 | 3.2 |
| 350 sq ft per gallon | 1.9 | 2.7 |
| 400 sq ft per gallon | 1.7 | 2.4 |
Mistake 4: forgetting the ceiling
The ceiling is the same area as the floor, and in a 12 × 12 ft room that is 144 sq ft more. Gallons by room size, two coats:
| Room | Walls only | Walls and ceiling |
|---|---|---|
| 10 × 10 ft | 1.5 | 2.1 |
| 12 × 12 ft | 1.9 | 2.7 |
| 12 × 16 ft | 2.3 | 3.4 |
| 14 × 20 ft | 2.8 | 4.4 |
Mistake 5: skipping the prep
- Fill holes and sand them flat, and wipe the dust off, or the patches show through the paint.
- Use painters tape on trim, and pull it while the paint is still slightly wet for a clean line.
- Mix every can together in one bucket (boxing) so that tiny differences in color do not show between cans.
- Cut in the edges first with a brush, then roll the wall while the cut-in is still wet.
For brushes, rollers, tape and primer along with the paint, see the room painting supplies calculator, and the exterior paint calculator for the outside. Estimates only.
Frequently asked questions
How much paint do I need for a 12 × 12 ft room?
With an 8 ft ceiling, one door, two windows and two coats, about 1.9 gallons for the walls. Add the ceiling and it is 2.7.
Do I paint the ceiling the same as the walls?
No. Ceilings usually take a flat, ceiling paint, so buy it separately and count it as its own job.
Should I buy a bit extra?
Yes, for touch-ups, and buy it all at once so the color matches. Rounding up to the next full gallon covers most of this.
Do I always need two coats?
Not always, but a color change or fresh drywall usually does. One coat of primer first helps with both. See the table above for the difference.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.