How to Calculate Paint
Estimating paint correctly saves money and prevents extra trips to the store. The key is calculating the total wall area, subtracting openings, and accounting for the coverage rate of your chosen paint.
Wall Area Formula
For a rectangular room, calculate the total wall area by finding the perimeter and multiplying by the ceiling height:
Wall Area=Perimeter×Height−Openings
Gallons Needed
Once you know the net paintable area, divide by the coverage rate and multiply by the number of coats:
Gallons=Net Area×CoatsCoverage Rate
Standard Openings
Use these typical sizes when subtracting openings: standard door is 21 sq ft (3' × 7'), standard window is 12 sq ft (3' × 4'), and a sliding glass door is about 40 sq ft.Coverage Rates by Surface
Different surfaces absorb paint at different rates. Rough or porous surfaces require more paint per square foot:
| Surface Type | Coverage (sq ft/gal) | Coats Needed |
|---|---|---|
| Smooth Drywall | 350-400 | 2 |
| Textured Drywall | 250-300 | 2 |
| Bare Wood | 250-350 | 2 + primer |
| Brick / Stucco | 150-250 | 2 + primer |
| Previously Painted | 350-400 | 1-2 |
Color Change Matters
When making a drastic color change (e.g., dark to light or light to dark), always use a tinted primer first. Without primer, you may need 3-4 coats instead of 2, which actually costs more than buying a separate primer.