Paint is sold by the gallon, so the goal is to turn your room into a number of gallons. Three things drive it: the wall area, the number of coats, and how far a gallon covers.
Step 1 — wall area
Add up the wall lengths (the perimeter) and multiply by the ceiling height:
wall area (ft²) = 2 × (length + width) × ceiling heightA 12 × 14 ft room with 8 ft ceilings has 2 × (12 + 14) × 8 = 416 ft² of wall.
Step 2 — coats and coverage
Two coats is the standard for an even finish, especially over a color change. A gallon covers about 350–400 ft² per coat on smooth, primed drywall — use 350 to be safe.
gallons = wall area × coats ÷ coverage (round up)For our room: 416 × 2 ÷ 350 = 2.38, so buy 3 gallons.
Textured or bare walls drink paint. Knockdown, orange-peel, and unprimed drywall can drop coverage to ~250 ft²/gallon — and usually want a primer coat first. Plan for more.
Should you subtract doors and windows?
You can, but many painters don’t — leaving small openings in builds a handy
buffer for cut-in and touch-ups. If you want to deduct, use about 20 ft² per
door and 15 ft² per window. Painting the ceiling adds length × width
to the area.
Worked examples (2 coats, 350 ft²/gal)
| Room | Wall area | Gallons |
|---|---|---|
| 10 × 10, 8 ft | 320 ft² | 2 |
| 12 × 14, 8 ft | 416 ft² | 3 |
| 14 × 16, 9 ft | 540 ft² | 4 (a 5-gal pail is cheaper) |
Frequently asked questions
How many square feet does a gallon of paint cover? About 350–400 ft² per coat on smooth drywall; ~250 on textured or bare surfaces.
Do I really need two coats? Usually yes — one coat rarely hides evenly, and a big color change can need three. “One-coat” paints still look better with two.
Is it cheaper to buy a 5-gallon pail? Per gallon, yes — once you need about 4+ gallons, a pail usually beats individual cans.
Get the exact number for your room — including coats, surface, and the ceiling — with the Paint Calculator.