How the Paint Calculator Works
The calculator first finds the room’s wall perimeter (2 × length + 2 × width) and multiplies it by wall height to get the gross wall area. It then subtracts a standard estimated area for each door (about 20 sq ft) and window (about 15 sq ft), since those surfaces aren’t painted the same way as open wall.
Coats and Coverage
The net paintable area is multiplied by the number of coats you plan to apply — most rooms look best with two coats, especially over a different existing color. That total is then divided by your paint’s coverage rate (a typical gallon of interior paint covers roughly 350-400 square feet per coat, though this varies by brand and paint type) to estimate the gallons and liters needed.
Tips
- Check the coverage rate printed on your specific paint can for a more precise estimate.
- Round up to the nearest full gallon or liter when purchasing, and consider buying slightly extra for touch-ups.
- Textured or porous walls (like new drywall or masonry) typically need more paint than the standard coverage rate assumes.
Provided for general planning and educational purposes only.