Interior Paint Calculator (by Room Size)
Measure the room's length, width and ceiling height. The calculator finds the wall area, subtracts doors and windows, multiplies by the number of coats and divides by the coverage on the can. Then it tells you what sizes to buy.
Formula
Wall area = 2 × (length + width) × height − (doors × 21 sq ft) − (windows × 15 sq ft). Gallons = area × coats ÷ coverage. Most premium interior paints list about 350–400 sq ft per gallon per coat on smooth, primed drywall (check your label). Use 250–300 for textured walls, or new drywall that hasn't been primed.
Paint for common rooms (8 ft ceiling, 1 door, 2 windows, 2 coats)
| Room | Wall sq ft | Wall gallons | Buy (walls) | Buy (ceiling) |
|---|---|---|---|---|
| 10 × 10 ft | 269 | 1.54 | 2 gal | 1 gal |
| 10 × 12 ft | 301 | 1.72 | 2 gal | 1 gal |
| 12 × 12 ft | 333 | 1.9 | 2 gal | 1 gal |
| 12 × 14 ft | 365 | 2.09 | 2 gal + 1 qt | 1 gal |
| 14 × 16 ft | 429 | 2.45 | 2 gal + 2 qt | 1 gal + 2 qt |
| 16 × 20 ft | 525 | 3 | 3 gal | 2 gal |
When you'll need more
- Big color changes (dark to light, or red/yellow) can take a third coat or a tinted primer.
- Fresh drywall soaks up paint, so always prime with PVA first.
- Buy a gallon instead of 3 quarts. It usually costs about the same, and the extra is good for touch-ups.
Frequently asked questions
How much paint for a 12x12 room?
With 8 ft ceilings, one door and two windows, the walls are about 333 sq ft. Two coats at 350 sq ft/gal is 1.9 gallons, so buy 2 gallons. The ceiling needs about 1 more gallon.
How many square feet does a gallon of paint cover?
Typically 350–400 sq ft per coat on smooth, primed walls, per manufacturer labels. Textured or porous surfaces get less.
Do I need two coats of paint?
Usually, yes. Two coats give even color and sheen. Some paint-and-primer products cover in one over a similar color.
Last reviewed October 2026 by Jim Frith. These are planning estimates; confirm quantities with your supplier and follow local codes.