Buying paint comes down to one number: how much wall you actually have to cover. Here's the simple method — including the doors, windows and coats most people forget — with a worked example.
Step 1: find the wall area
For a rectangular room, the wall area is 2 × (length + width) × ceiling height. For a 12 × 12 room with 8-foot ceilings that's 2 × 24 × 8 = 384 square feet before you take anything out.
Step 2: subtract doors and windows
You don't paint the openings, so subtract them. Use about 21 square feet per door and 15 square feet per window. One of each drops our room to 384 − 36 = 348 square feet.
Step 3: multiply by coats, divide by coverage
A gallon covers roughly 350 square feet per coat on smooth walls. Two coats is standard, so:
- 348 × 2 coats = 696 square feet of coverage needed
- 696 ÷ 350 = 1.99 gallons → buy 2 gallons
Always round up to whole cans. Running out halfway up a wall leaves visible lap marks that are hard to hide.
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Open the paint calculator →How far does a gallon really go?
Manufacturers quote 350–400 square feet per gallon, and that holds on smooth, primed drywall. New drywall, textured surfaces, and dramatic color changes (light over dark) all drink more paint — which is exactly why two coats is the sensible default.
One or two coats?
Two coats is standard for an even, durable finish. Go to three when you're painting a bold color, going light over dark, or covering fresh drywall that hasn't been primed.
Don't forget the ceiling
This method covers the walls. For the ceiling, calculate its area separately (length × width) and add it if you're painting it too.
Quick answers
How much paint for a 12×12 room? About 2 gallons for two coats, with one door and one window.
How many square feet per gallon? About 350–400 per coat on smooth walls.
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