One gallon of interior paint covers 350–400 square feet per coat on smooth walls. This range reflects real-world variability — surface texture, paint sheen, color opacity, and application method all affect how far a gallon goes. Use both numbers as a planning range rather than a single estimate.
The formula
gallons needed = (wall area × number of coats) ÷ coverage rate
The coverage rate is 350–400 sq ft/gallon. Use 400 for smooth, primed walls with a light color; use 350 for textured walls, dark-to-light color changes, or unprimed drywall.
How to measure wall area
- Measure each wall's width and height in feet, then multiply.
- Add up all four walls.
- Subtract approximately 21 sq ft per door and 15 sq ft per window — most painters skip this step for small rooms since the overlap covers edge cutting waste.
Example — 12 × 12 room with 8-foot ceilings:
4 walls: (12 + 12 + 12 + 12) × 8 = 384 sq ft
Subtract 1 door (21 sq ft) and 1 window (15 sq ft): 384 − 36 = 348 sq ft
At 2 coats: 348 × 2 = 696 sq ft ÷ 400 = 1.74 gallons (economy estimate), or 696 ÷ 350 = 1.99 gallons (generous). Buy 2 gallons.
What affects actual coverage
Surface texture: Smooth, primed walls absorb less paint. Textured or unprimed drywall can absorb 20–30% more, pushing coverage toward 300–350 sq ft/gallon.
Color change: Painting over a dark color with a lighter one typically requires an extra coat. Factor that into your coat count.
Paint sheen: Flat/matte paints tend to spread slightly better than semi-gloss or high-gloss, though the difference is small compared to surface prep.
Application method: Rollers leave a thicker film than brushes; sprayers apply thinner coats. Adjust your estimate if using a sprayer.
How much extra paint to buy
Always buy one additional quart (0.25 gallon) beyond your estimate. Keep it sealed, cool, and labeled with the room name and paint color code. Touch-up paint from the same batch is the only way to guarantee a seamless match for future scuffs and nail holes.
Quick reference
| Room size | Wall area (est.) | 1 coat | 2 coats |
|---|---|---|---|
| Small bathroom (5×8) | ~105 sq ft | ~0.3 gal | ~0.6 gal |
| Bedroom (10×12) | ~352 sq ft | ~1 gal | ~2 gal |
| Living room (14×18) | ~512 sq ft | ~1.4 gal | ~2.8 gal |
| Open plan (20×24) | ~704 sq ft | ~1.9 gal | ~3.8 gal |