Quick Answer
Most interior paint covers about 350 square feet per gallon per coat. To estimate how much paint you need, calculate the surface area, subtract large openings, multiply by the number of coats, then divide by the coverage per gallon.
Paint needed = Paintable square feet × Coats ÷ Coverage per gallon
For example, a room with 800 paintable square feet and two coats needs:
800 × 2 ÷ 350 = 4.6 gallons, so you would usually buy 5 gallons.
How to Calculate Paint for a Room
- Measure each wall's width and height.
- Multiply width × height for each wall.
- Add the wall areas together.
- Subtract large windows and doors if you want a tighter estimate.
- Multiply by the number of coats.
- Divide by paint coverage per gallon.
| Surface | Typical Coverage |
|---|---|
| Smooth interior wall | 350-400 sq ft/gallon |
| Textured wall | 250-325 sq ft/gallon |
| New drywall or primer coat | 250-350 sq ft/gallon |
| Exterior siding | 250-350 sq ft/gallon |
| Rough stucco or brick | 150-250 sq ft/gallon |
How Many Coats?
Use one coat for small touch-ups or when repainting with a very similar color. Use two coats for most room painting. Use primer plus two coats for new drywall, stains, dark-to-light color changes, or uneven surfaces.
Dark colors, bright colors, and cheap paint often need more coverage than the label suggests. If you are close to the next gallon, buy the extra gallon rather than running short mid-wall.
Ceilings, Doors, and Windows
A ceiling is painted just like a floor: length × width. A 12 × 12 ft ceiling is about 144 square feet, or roughly 0.4 gallon for one coat at standard coverage. In the calculator above, check include ceiling and that area is added automatically.
Doors and windows work the other way. The calculator treats them as openings and subtracts their area from the walls — about 20 sq ft per door and 15 sq ft per window — because you do not paint over that part of the wall. Enter the number of doors and windows so the estimate removes the area you will not roll.
Key Takeaways
- Use 350 sq ft per gallon as a practical interior estimate.
- Multiply by the number of coats before dividing by coverage.
- Rough or exterior surfaces need more paint.
- Buy a little extra for touch-ups and color matching.
- Use the calculator above for rooms, walls, ceilings, and exterior estimates.