Paint Guide: Everything You Need to Estimate
To estimate paint, find the 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 12×12 ft room with 8 ft walls needs about 2 gallons for two coats, costing roughly $50–$120.
Get exact numbers
Use the free Paint Calculator to estimate your project.
How to estimate paint
Formula: Gallons = (Wall area − openings) × coats ÷ 350 sq ft per gallon
- Measure the walls. Add all wall lengths (perimeter) and multiply by ceiling height.
- Subtract openings. Remove ~21 sq ft for each standard door or window.
- Multiply by coats. Most jobs need two coats for even coverage.
- Divide by coverage. One gallon covers roughly 350 sq ft per coat.
Example: A 12×12 ft room, 8 ft walls, 2 openings: perimeter 48 ft × 8 = 384 ft² − 42 = 342 ft² × 2 coats = 684 ÷ 350 ≈ 1.95 gallons.
Common mistakes to avoid
Buying for one coat
Most walls need two coats for even color. Estimate for two unless you're repainting the same color over a clean surface.
Not subtracting openings
Large windows and doors reduce the paintable area — but don't over-subtract small trim, or you'll come up short.
Ignoring surface texture
Textured, porous, or unprimed walls drink paint and cover less than 350 sq ft per gallon. Round up.
Skipping primer
Bare drywall and big color changes need primer — a separate product from your finish coats.