Paint Calculator
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.
- Coverage per gallon
- ~350 ft²
- Standard coats
- 2
- Per door/window
- ~21 ft²
Your measurements
Each opening subtracts ~21 sq ft of wall.
Enter your measurements
Fill in the fields to see your estimate.
The formula
Gallons = (Wall area − openings) × coats ÷ 350 sq ft per gallonHow to calculate it manually
- 1
Measure the walls
Add all wall lengths (perimeter) and multiply by ceiling height.
- 2
Subtract openings
Remove ~21 sq ft for each standard door or window.
- 3
Multiply by coats
Most jobs need two coats for even coverage.
- 4
Divide by coverage
One gallon covers roughly 350 sq ft per coat.
Example calculation
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.
Paint needed by room size
Gallons for two coats on a room with 8 ft walls, minus two openings.
| Room size | Wall area | Gallons (2 coats) | Paint cost |
|---|---|---|---|
| 10 × 10 ft | ~318 ft² | ~2 gal | $50 – $120 |
| 12 × 12 ft | ~342 ft² | ~2 gal | $50 – $120 |
| 15 × 15 ft | ~438 ft² | ~3 gal | $75 – $180 |
| 20 × 20 ft | ~598 ft² | ~4 gal | $100 – $240 |
Common paint estimating mistakes
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.
Frequently asked questions
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Ready to start your painting project?
Run the numbers above, then explore related tools and guides to plan every step.