Paint is estimated from the wall area, the number of coats, and a known coverage rate per gallon. Get those three and the gallons fall out of a simple division. The most common mistakes are using floor area instead of wall area, and forgetting the second coat.
Step 1: find the wall area
Wall area is the room perimeter times the ceiling height, not the floor area. Add up the length of all the walls, then multiply by the height. A 12 by 12 room has a perimeter of 48 feet, and with 8 foot walls that is 384 square feet of wall. A 12 by 16 room has a 56 foot perimeter, so 448 square feet at 8 feet.
Step 2: apply coverage and coats
One gallon of interior latex covers about 350 to 400 square feet in a single coat. Use 375 as a working average. Multiply your wall area by the number of coats, then divide by 375.
Worked example
That 12 by 12 room at 384 square feet, two coats: 384 times 2 is 768, divided by 375 is about 2 gallons. Buy two gallons, or two gallons plus a quart if you want a margin for touch ups. Most rooms land between one and three gallons for two coats.
Coverage reference
| Surface | Coverage per gallon per coat |
|---|---|
| Smooth, previously painted wall | 375 to 400 sq ft |
| New or patched drywall | 300 to 350 sq ft |
| Rough or textured surface | 250 to 300 sq ft |
Ceilings and trim
A ceiling is just another surface: its area is the room length times width. Estimate it separately, usually with a dedicated ceiling paint. Trim, doors and baseboards use far less and are usually bought as a separate quart or gallon of a different sheen, so do not try to fold them into the wall figure.
Primer and deductions
Add primer for bare drywall, patched areas, or when going from a dark colour to a light one. You can leave normal doors and windows in the wall area as a built in buffer, but subtract large openings such as a garage door or a wall of glass. When in doubt, leave the openings in, since a little extra paint is cheaper than a second store trip mid job.
What changes coverage
- Rough, porous or previously unpainted surfaces drink more paint.
- Deep or saturated colours often need an extra coat to look even.
- Sprayed application can use more than a roller for the same area.
- Textured walls have more real surface than their flat dimensions suggest.