Enter the size of the room, the number of doors and windows, and how many coats you plan. The calculator works out the wall area to paint and turns it into gallons and quarts, using the coverage printed on your can.
How to use the paint calculator
- Measure the length and width of the room and the wall height from floor to ceiling.
- Count the doors and windows. Each door takes off 21 sq ft and each window 15 sq ft; for very large windows or patio doors, measure them and add them as extra windows.
- Enter the number of coats. A color change or new drywall usually needs 2 coats; a fresh coat of the same color may need only 1.
- Set the coverage from your paint can. Choose whether to include the ceiling.
Paint formula
Wall area = 2 × (length + width) × height − doors × 21 − windows × 15 Area to paint = wall area (+ length × width for the ceiling) Gallons = area to paint × coats ÷ coverage per gallon
Worked example: a 14 × 12 ft bedroom
- Wall area: 2 × (14 + 12) × 8 = 416 sq ft, minus one door (21) and two windows (30) = 365 sq ft.
- Two coats: 365 × 2 = 730 sq ft.
- At 350 sq ft per gallon: 730 ÷ 350 = 2.09 gallons.
- Buy 2 gallons + 1 quart.
Paint needed by room size
Walls only, 8 ft ceilings, one door and two windows, 2 coats at 350 sq ft per gallon:
| Room | Wall area | Paint (exact) | Buy |
|---|---|---|---|
| 10 × 10 ft | 269 sq ft | 1.54 gal | 1 gallon + 3 quarts |
| 12 × 10 ft | 301 sq ft | 1.72 gal | 1 gallon + 3 quarts |
| 12 × 12 ft | 333 sq ft | 1.9 gal | 2 gallons |
| 14 × 12 ft | 365 sq ft | 2.09 gal | 2 gallons + 1 quart |
| 16 × 14 ft | 429 sq ft | 2.45 gal | 2 gallons + 2 quarts |
| 20 × 15 ft | 509 sq ft | 2.91 gal | 3 gallons |
How many square feet does a gallon of paint cover?
Sherwin-Williams says a gallon typically covers about 350–400 sq ft. Rough or porous surfaces such as new drywall, textured plaster and bare wood soak up more paint, so use the low end of the range, or prime first. Dark colors over light walls, or light over dark, often need an extra coat.
Tips
- Buy a little extra of the same batch for touch-ups; tinted paint can vary between batches.
- Trim, doors and cabinets are usually a different sheen; estimate them separately.
- Primer is estimated the same way, using the coverage on the primer can.
Questions
How much paint do I need for a 12 × 12 room?
About 1.9 gallons for two coats on the walls, so 2 gallons, with an 8 ft ceiling, one door and two windows.
Should I subtract doors and windows?
Yes for an accurate figure. If you are close to a whole gallon, rounding up covers touch-ups.
Does the ceiling need different paint?
Ceilings are usually painted with flat ceiling paint. Choose the ceiling option to add its area, or calculate it separately if you use a different product.
Sources
- Paint Calculator, Sherwin-Williams.
Used for: Coverage of about 350–400 sq ft per gallon. - NIST Guide to the SI, Appendix B.8: Factors for units listed alphabetically, National Institute of Standards and Technology.
Used for: Exact foot, inch and metre definitions.