How much paint you need
Wall area = 2 × (length + width) × height − doors × 21 − windows × 15
Paint area = (wall area [+ ceiling length × width]) × coats
Gallons = paint area ÷ coverage per gallon
Assumptions: a door is 3 × 7 ft (21 sq ft) and a window 3 × 5 ft (15 sq ft). Sherwin-Williams states that a gallon typically covers about 350–400 sq ft, a quart about 100 sq ft, and a gallon of primer about 200–300 sq ft. We default to the low end, 350, because rough or porous surfaces and dark-to-light color changes absorb more. Use the coverage printed on your product’s label if it differs.
To avoid buying a whole extra gallon, a remainder of three quarts or less is rounded up to quarts; four quarts become a gallon.
Worked example
A 14 × 12 ft room with 8 ft walls, one door and two windows, two coats at 350 sq ft/gal:
| Walls: 2 × (14 + 12) × 8 | 416 sq ft |
|---|---|
| Minus 1 door and 2 windows (21 + 30) | 365 sq ft |
| Two coats | 730 sq ft |
| Gallons (730 ÷ 350) | 2.09 |
| Buy | 2 gallons + 1 quart |
Tips for accurate estimates
- Two coats is standard for a color change; one may do for a refresh in the same color.
- Textured walls, bare drywall and masonry absorb more — lower the coverage figure or prime first.
- Keep a little extra from the same batch for touch-ups.
Frequently asked questions
How many square feet does a gallon of paint cover?
Sherwin-Williams says a gallon typically covers about 350–400 square feet, and a quart about 100 square feet. Coverage drops on rough or porous surfaces.
How much paint for a 12x12 room?
With 8 ft walls, one door and two windows, the walls are about 333 sq ft; two coats need about 1.9 gallons at 350 sq ft per gallon, so buy 2 gallons.