The Standard Paint Coverage Rule
One gallon of interior latex paint covers approximately 350–400 square feet of smooth wall surface per coat. If the walls have a heavy texture (knockdown, orange peel, or skip-trowel), coverage drops to 300–350 sq ft per gallon because the textured surface has more total area and absorbs more paint. Always check the coverage printed on the specific paint can you buy — premium paints with higher solid content sometimes cover up to 450 sq ft per gallon.
Step 1: Calculate Total Wall Area
Measure the perimeter of the room (all four walls added together) and multiply by the ceiling height:
- Measure all four walls and add their lengths: e.g., 12 ft + 15 ft + 12 ft + 15 ft =
54 linear feet. - Multiply by ceiling height (standard rooms = 8 feet):
54 × 8 = 432 sq ft of gross wall area. - Subtract doors (average 21 sq ft each) and windows (average 15 sq ft each):
432 – 21 – 30 = 381 sq ft net wall area.
Step 2: How Many Gallons Per Coat?
Divide your net wall area by the paint's coverage rate:
381 sq ft ÷ 350 sq ft/gallon = 1.09 gallons per coat
For most painting projects, two coats are required for even color and proper opacity. This is especially true when painting a dark color over a lighter one, or vice versa. Total: 1.09 × 2 = 2.18 gallons → buy 3 gallons (always round up to the nearest whole gallon or half-gallon).
When Do You Need Primer?
Primer is a separate coat that seals the surface, improves adhesion, and reduces the number of finish coats needed. Use primer when:
- Painting over bare drywall or fresh plaster
- Making a dramatic color change (e.g., deep red to soft white)
- Covering water stains, smoke damage, or oil-based paint
- Using a very dark or very bright finish color
Primer coverage is similar to paint: approximately 300–400 sq ft per gallon. Calculate primer needs the same way as your first coat of paint.
Ceiling Paint Calculation
To paint the ceiling, simply calculate the floor area: Room Length × Room Width. For a 12 ft × 15 ft room, the ceiling is 180 sq ft. At 350 sq ft/gallon, one gallon covers this ceiling in one coat. If applying two coats: 2 gallons needed.
Paint Quantity Reference Table
| Room Size | Net Wall Area (approx.) | Gallons (1 coat) | Gallons (2 coats) |
|---|---|---|---|
| Small Bedroom (10×10) | ~280 sq ft | 1 gal | 2 gal |
| Average Bedroom (12×14) | ~370 sq ft | 1 gal | 3 gal |
| Living Room (16×20) | ~550 sq ft | 2 gal | 4 gal |
| Open-Plan (20×30) | ~760 sq ft | 3 gal | 5 gal |
Tips to Avoid Wasting Paint
- Use the right finish: Flat/matte for ceilings and low-traffic walls. Eggshell or satin for high-traffic areas. Semi-gloss for trim, bathrooms, and kitchens.
- Keep leftover paint: Store in an airtight container for touch-ups. Label the can with the room name and color code.
- Cut in before rolling: Always paint edges with a brush before rolling the main wall area — this prevents re-rolling and saves paint.
Use our free Paint Calculator to get an instant gallon estimate for any room dimensions you enter.