How the paint calculator works
The formulas, default values and sources behind every result.
Last updated September 20, 2026
Formulas
Openings = doors × door size + windows × window size
Paintable area = wall area − openings
Wall paint (gal) = paintable area × coats ÷ coverage × (1 + waste)
Ceiling paint (gal) = length × width × ceiling coats ÷ 350 × (1 + waste)
Primer (gal) = paintable area ÷ 250 × (1 + waste)
Amounts are rounded up to the next quart. The calculator does not include trim, doors, baseboards or closets, and it does not handle exterior surfaces.
Default values
| Setting | Default | Where it comes from |
|---|---|---|
| Coverage, smooth painted walls | 400 sq ft/gal | Top of the 350 to 400 range given by retailers |
| Coverage, typical walls (default) | 350 sq ft/gal | Bottom of the same range, a cautious default |
| Coverage, textured walls | 325 sq ft/gal | Middle of the 300 to 350 range |
| Coverage, new drywall or porous | 275 sq ft/gal | Within the 250 to 300 range |
| Ceiling coverage | 350 sq ft/gal | Within the 300 to 400 range |
| Primer coverage | 250 sq ft/gal | Middle of the 200 to 300 range |
| Door size | 21 sq ft | A standard 3 × 7 ft door |
| Window size | 15 sq ft | A standard 3 × 5 ft window |
| Extra for waste | 10% | A commonly recommended allowance |
Published guidance varies, so these are our chosen values within the published ranges. Some sources deduct 20 sq ft for a door and 10 sq ft for a window, which is why other calculators can give slightly different results. You can change the door size, window size and waste allowance under Advanced settings in the calculator, and you can enter the coverage from your own paint can.
Sources
- Lowe's paint calculator: paint coverage of 350 to 400 sq ft per gallon and primer at 200 to 300.
- This Old House: method for figuring wall area, coats and openings.
- CalculatorSoup: 21 sq ft for a door and 15 sq ft for a window, and coverage for textured surfaces.
- RemodelCalculators: coverage on textured walls and bare drywall, and the 10% allowance for waste.
- Caesar Cipher: ceiling coverage and one coat for most ceilings.
Limits of the estimate
- The result is an estimate. The coverage printed on your paint can and your own application method will change how much you use.
- The calculator assumes a rectangular room. For other shapes, calculate each part separately.
- Prices vary by store and region. The optional cost estimate simply multiplies your price per gallon by the amount to buy.
Keeping it accurate
We review the assumptions and sources on this page from time to time. If you find a mistake or a better source, write to YOUR-EMAIL@YOURDOMAIN.COM.