Buying the right amount avoids a second trip to the store, and avoids paint from a different batch that may not match the first.
How to use the paint calculator
- Measure the length and width of the room and the wall height.
- Count the doors and windows. The calculator subtracts about 20 sq ft per door and 15 sq ft per window.
- Choose how many coats. Two coats is standard for an even finish.
- Choose whether to include the ceiling, then check the result.
The formula
Wall area: 2 × (length + width) × height − (doors × 20) − (windows × 15).
Gallons: paint area × coats ÷ coverage. Most paint covers 350–400 sq ft per gallon on smooth walls.
Paint needed for common room sizes (8 ft walls, 1 door, 2 windows, 2 coats, walls only)
| Room size | Gallons needed | Buy |
|---|---|---|
| 10 × 10 ft | 1.5 | 2 gal |
| 12 × 12 ft | 1.9 | 2 gal |
| 12 × 14 ft | 2.1 | 3 gal |
| 14 × 16 ft | 2.5 | 3 gal |
| 15 × 20 ft | 2.9 | 3 gal |
| 20 × 20 ft | 3.4 | 4 gal |
Frequently asked questions
How much does one gallon of paint cover?
About 350 to 400 square feet with one coat on smooth, primed walls. Textured or porous surfaces cover less.
How much paint do I need for a 12×12 room?
With 8 ft walls, one door, two windows and two coats, about 2 gallons for the walls.
Do I really need two coats?
Usually yes. A second coat gives even color and better durability, especially when changing colors. Paint-and-primer products sometimes cover in one coat over a similar color.
Should I buy a 5-gallon bucket?
If you need 4 or more gallons of the same color, a 5-gallon bucket is usually cheaper per gallon and guarantees the color matches throughout.
Results are estimates for planning and budgeting. Always confirm measurements and product coverage before you buy, and follow local building codes.