How the paint math works
- Wall area. Perimeter × height: 2 × (length + width) × height.
- Subtract openings. Doors × door size plus windows × window size.
- Add the ceiling if you're painting it with the same paint: length × width.
- Multiply by coats, divide by coverage. Sherwin-Williams puts a gallon at about 350–400 sq ft; we use 350 unless you change it.
- Round to what the store sells. Whole gallons, plus a quart if you're only a quart over.
Example: a 12 × 12 ft room with 8 ft walls has 2 × 24 × 8 = 384 sq ft of wall. Minus one door (21) and two windows (30) leaves 333 sq ft. Two coats is 666 sq ft; at 350 sq ft per gallon that's 1.9 gallons, so buy 2 gallons.
Paint by room size
Two coats, one door, two windows, 350 sq ft per gallon.
| Room | Wall area | Walls only | Walls and ceiling |
|---|---|---|---|
| 10 × 10 ft, 8 ft walls | 269 sq ft | 1.5 gal | 2.1 gal |
| 12 × 12 ft, 8 ft walls | 333 sq ft | 1.9 gal | 2.7 gal |
| 12 × 14 ft, 8 ft walls | 365 sq ft | 2.1 gal | 3 gal |
| 14 × 16 ft, 8 ft walls | 429 sq ft | 2.5 gal | 3.7 gal |
| 12 × 12 ft, 9 ft walls | 381 sq ft | 2.2 gal | 3 gal |
| 16 × 20 ft, 9 ft walls | 597 sq ft | 3.4 gal | 5.2 gal |
Painting tips
- Ceilings are often painted with a different, flat paint. If so, run the calculator twice: walls only, then the ceiling area alone.
- Keep a little left over for touch-ups, labeled with the room and date.