The formula
Wall area = 2 × (length + width) × height − doors × 20 − windows × 15 Gallons = wall area × coats ÷ coverage per gallon
Example: a 12 × 14 ft room with 8 ft walls, one door and two windows has 2 × 26 × 8 = 416 square feet of wall, minus 20 and 30 = 366 square feet. Two coats at 350 square feet per gallon need 2.09 gallons, so buy 2 gallons and 1 quart.
Quick answers for common rooms
| Room, 8 ft walls | Wall area | Paint, 2 coats |
|---|---|---|
| 10 × 10 ft | 285 sq ft | 1.6 gal |
| 10 × 12 ft | 317 sq ft | 1.8 gal |
| 12 × 12 ft | 349 sq ft | 2 gal |
| 12 × 14 ft | 381 sq ft | 2.2 gal |
| 14 × 16 ft | 445 sq ft | 2.5 gal |
How far a gallon goes
Most interior paint covers 350 to 400 square feet per gallon per coat; the exact figure is on the can. Textured walls and bare drywall soak up more, so use a lower number for them.
Tips
- Two coats is standard. Bold colours over a different shade may need a third coat or a tinted primer.
- Keep some leftover paint for touch-ups; a matched batch is hard to get later.
Sources and assumptions
Coverage of 350 to 400 square feet per gallon is the range most paint makers print on the can. Doors are counted as 20 square feet and windows as 15, typical sizes for interior doors and windows.
Questions
How much does a gallon of paint cover?
About 350 to 400 square feet with one coat on smooth walls.
How much paint do I need for a 12 × 12 room?
With 8-foot walls, one door and one window there are about 349 square feet of wall, so roughly 2 gallons for two coats.
Do I need two coats of paint?
Usually yes. One coat rarely hides the old colour evenly, especially when you change colours.