How the paint calculator works
- Wall area = perimeter (2 × length + 2 × width) × wall height.
- Subtract about 21 sq ft (1.95 m²) for each door and 15 sq ft (1.4 m²) for each window.
- Multiply by the number of coats and divide by the paint’s coverage.
A 12 × 12 ft room with 8 ft walls has 384 sq ft of wall. After one door and two windows, about 333 sq ft remains. Two coats at 350 sq ft per gallon need about 1.9 gallons — so buy 2 gallons.
Paint coverage
| Surface | Coverage per gallon (per litre) |
|---|---|
| Smooth, previously painted walls | 350–400 sq ft (9–10 m²) |
| New drywall (after primer) | 300–350 sq ft |
| Textured walls, popcorn ceilings | 200–300 sq ft (5–7 m²) |
| Brick, block, rough stucco | 100–200 sq ft |
Check the paint can — manufacturers list their coverage. Dark colors over light walls (or the reverse) often need a third coat or a tinted primer.
How many coats?
- Two coats is the standard for an even, durable finish.
- One coat can work when repainting the same color with a quality paint.
- Primer first on new drywall, bare wood, stains, glossy surfaces or big color changes.
Buying tips
A US gallon is about 3.79 litres and a quart is a quarter gallon. Buy enough in one batch so the color matches, and keep a little for touch-ups. Doing floors too? Use the tile calculator or square footage calculator.
Frequently asked questions
How much paint do I need for a 12×12 room?
About 2 gallons for two coats on the walls with 8 ft ceilings, one door and two windows.
How many square feet does a gallon of paint cover?
Typically 350–400 square feet on smooth walls for one coat.
Do I need two coats of paint?
Usually yes, for even color and durability — especially with a color change.
How much paint for a ceiling?
Ceiling area is length × width. A 12×12 ceiling (144 sq ft) needs under half a gallon per coat.