Paint guide
How much paint do I need?
Start with the net wall area, decide how many coats you expect to apply, and use the product’s stated coverage for a planning estimate. Then round up to whole cans.
1. Calculate the wall surface
Multiply the room perimeter by the wall height. Subtract the measured area of doors and windows. A 12 × 10 ft room with 8 ft walls has 352 sq ft of gross wall area; subtract 20 sq ft of openings for 332 sq ft to paint. The wall measuring guide covers the details.
2. Multiply by coats and divide by coverage
Planning gallons = gallons before allowance × (1 + allowance ÷ 100)
At 350 sq ft per gallon per coat, the 332 sq ft room needs 0.95 gal per coat, 1.90 gal for two coats, and 2.09 gal with a 10% allowance. Rounded up, that is 3 gallons, with roughly 0.9 gal left in the last can.
3. Check the coverage on your can
Coverage is a planning input, not a constant. Paint, surface texture, color change, application method, and the manufacturer’s label all change the final amount. Sherwin-Williams describes typical interior paint coverage of about 350–400 sq ft per gallon; porous new drywall and rough textures sit lower.
Quick reference
| Room (8 ft walls) | Wall area | Buy |
|---|---|---|
| 10 × 10 ft | 320 sq ft | 3 gal |
| 12 × 12 ft | 384 sq ft | 3 gal |
| 12 × 14 ft | 416 sq ft | 3 gal |
| 14 × 16 ft | 480 sq ft | 4 gal |
| 16 × 20 ft | 576 sq ft | 4 gal |
Wall areas are gross; subtract your doors and windows in the calculator.
Before you buy
- Check whether the label’s coverage is per coat.
- Estimate the ceiling and trim separately; they often use different products.
- Add primer for new drywall, patches, or big color changes.
- Buy all the paint for one room at once to keep the tint batch consistent.
Reference: Sherwin-Williams paint calculator guidance