How Much Paint Do I Need? (Room Calculator Guide)
To estimate paint, calculate wall area (perimeter × height), subtract about 21 sq ft per door or window, multiply by the number of coats, and divide by 350 (sq ft per gallon). A standard 12×12 ft room needs about 2 gallons for two coats. Rough or unprimed surfaces cover less, so buy a little extra.
Buying paint is a balancing act: run short mid-wall and you risk a visible lap mark where a fresh can blends in, but over-buy and you've got half-used cans cluttering the garage. A quick calculation based on your wall area, coats, and openings gets it right.
This guide shows how to estimate how much paint you need for any room, the coverage you can expect from a gallon, and when to plan for one coat versus two.
Key takeaways
- Paint (gallons) = (wall area − openings) × coats ÷ 350.
- One gallon covers about 350–400 ft² per coat on smooth walls.
- Plan on two coats for even, durable color.
- Subtract ~21 ft² per door and ~15 ft² per window.
- Rough, porous, or unprimed surfaces need more paint.
Skip the math
Get instant numbers with the Paint Calculator.
How to calculate paint
Find the wall area by multiplying the room's perimeter by the ceiling height. Subtract roughly 21 square feet for each door and 15 for each window. Multiply the result by the number of coats, then divide by 350 — the square feet a gallon covers. For a 12 × 12 ft room with 8 ft ceilings, that's about 384 ft² of wall, or roughly 2 gallons for two coats.
Paint coverage rates
One gallon covers roughly 350–400 square feet per coat on smooth, primed walls. Textured, porous, or unprimed surfaces absorb more and cover less — sometimes as little as 250 ft² per gallon — so add a margin for those.
Paint needed by room size
| Room size | Wall area | Gallons (2 coats) |
|---|---|---|
| 10 × 10 ft | ~320 ft² | 2 |
| 12 × 12 ft | ~384 ft² | 2 |
| 15 × 20 ft | ~560 ft² | 3–4 |
| Whole avg home | — | 6–10 |
One coat or two?
- Two coats is standard for durable, even color.
- Use two coats over fresh drywall or a big color change.
- Prime bare drywall, patches, or stains before painting.
- Buy 5–10% extra so you have a dye-lot match for touch-ups.