The one calculation that matters
Paint quantity is wall area divided by the can's coverage, times the number of coats. Everything else is refining those three numbers.
Wall area is the room perimeter times ceiling height. A 12 × 12 ft room with 8 ft walls is (12 + 12) × 2 × 8 = 384 ft². Two coats at 350 ft²/gal is 384 ÷ 350 × 2 = 2.2 gallons — round up, so buy two gallons plus a quart, or just two gallons if you subtract the door and windows.
Coverage depends on the surface, not the room
| Surface / paint | Coverage per gallon |
|---|---|
| Smooth, sealed wall (standard interior) | ~350 ft² |
| Premium interior, second coat | 350–400 ft² |
| Rough or textured wall | 250–300 ft² |
| Bare drywall or fresh plaster | 200–250 ft² (prime first) |
| Exterior / masonry paint | 150–250 ft² |
Room size changes how much paint you use in total, not the coverage rate. A porous surface is what actually eats extra paint, so check the number on the can you buy.
Subtract the openings, then round up
- Doors are about 20 ft² each and average windows about 15 ft². Subtract them once your walls have two or more openings.
- Always round the can count up. Running half a gallon short mid-wall means a second store trip and a possible batch-color mismatch.
- Add primer as its own coat for bare, dark or patched walls — it is not optional if you want the topcoat to look even.
- Ceiling is length × width if you are painting it, added on top of the wall area.
Common questions
How much paint do I need for a room?
Measure the wall area, divide by the coverage on the can (about 350 sq ft per gallon), then multiply by the number of coats. A typical 12 by 12 room with 8 ft walls needs roughly 1.5 gallons for two coats.
Does one gallon really cover 350 square feet?
That is the figure for one coat on a smooth, sealed wall. Rough, porous or previously unpainted surfaces soak up more, so coverage can drop to 250 to 300 sq ft per gallon. Bare drywall also needs primer first.
How many coats should I plan for?
Two is standard. Use one only if you are refreshing the same color over a good surface. Going from dark to light, or covering a bold color, can take three coats plus a primer.
