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.
Assumptions and limitations
The estimate uses standard deductions of 21 square feet per door and 15 square feet per window. It excludes ceilings, trim, primer, and application loss; actual coverage varies with texture, color change, and product.
Frequently asked questions
How many gallons of paint do I need for a room?
Measure the walls, subtract large openings, multiply by the number of coats, and divide by the coverage on the paint can. The calculator performs those steps and rounds up to whole gallons.
Does a second coat double the paint needed?
For planning, yes. Actual second-coat usage can be slightly lower, but the full estimate helps avoid running short.