Short answer: one gallon of interior paint covers about 350–400 ft² per coat (9–10 m² per litre). Most bedrooms need about 2 gallons for two coats on the walls, plus 1 gallon for the ceiling.
Step 1 — Measure the walls
Add up the length of all walls (the room's perimeter) and multiply by the ceiling height:
wall area = 2 × (length + width) × height
A 12 × 14 ft room with 8 ft ceilings: 2 × (12 + 14) × 8 = 416 ft².
Step 2 — Subtract doors and windows
Use about 20 ft² per door and 12–15 ft² per window (1.9 m² and 1.2 m²). With one door and two windows: 416 − 20 − 24 = 372 ft².
Step 3 — Multiply by coats
Two coats is the professional standard: 372 × 2 = 744 ft² of coverage needed.
Step 4 — Divide by coverage and add waste
gallons = area × coats ÷ coverage × 1.10
744 ÷ 375 × 1.1 = 2.18 gallons. Buy two gallons and a quart, or three gallons if you want spare paint for touch-ups.
Coverage by surface
| Surface | ft² per gallon | m² per litre |
|---|---|---|
| Smooth, previously painted drywall | 350–400 | 9–10 |
| New drywall (primed) | 300–350 | 8–9 |
| Textured / knockdown walls | 250–300 | 6–7.5 |
| Popcorn ceiling | 150–250 | 4–6 |
| Exterior wood siding | 250–300 | 6–7.5 |
| Stucco / brick | 150–250 | 4–6 |
Quick reference: walls only, two coats
| Room (8 ft ceiling) | Net wall area | Paint |
|---|---|---|
| 10 × 10 ft | ~287 ft² | 1.7 gal |
| 12 × 12 ft | ~351 ft² | 2.1 gal |
| 12 × 14 ft | ~383 ft² | 2.2 gal |
| 15 × 20 ft | ~527 ft² | 3.1 gal |
Assumes one door and one window (~33 ft² deducted), 375 ft²/gal and 10% waste.
Common mistakes
- Forgetting the second coat — the most common reason for a second trip to the store.
- Using the floor area instead of the wall area. Walls are typically 3× the floor area.
- Ignoring surface texture. Rough or porous surfaces can use a third more paint.
- Buying from different batches. Mix ("box") cans together for large areas.