How the paint calculator works
For a rectangular room, wall area is the room perimeter multiplied by wall height. The calculator subtracts 21 square feet for each standard door and 15 square feet for each standard window, then multiplies the remaining area by the number of coats and your extra allowance.
Paint formula
Wall area = 2 × (length + width) × height. Estimated gallons are the adjusted paintable area divided by the coverage shown on the paint can, rounded up to a whole gallon.
Typical paint coverage
| Paint type / condition | Planning range |
|---|---|
| Smooth, previously painted wall | About 350–400 ft²/gal |
| Porous or textured surface | Often lower |
| Major color change | May require primer or extra coats |
Example
A 14 × 12 foot room with 9-foot walls has 468 square feet of wall surface before doors and windows are deducted. One standard door and two standard windows reduce that estimate by 51 square feet.
Tips before buying
- Use the coverage figure printed on your specific paint.
- Measure unusually large doors or windows separately.
- Very rough surfaces may need substantially more paint.
- Ceilings and trim are not included in this calculator.