How we worked it out
Updated as you change the numbers.
- Wall area = 2 × (length + width) × height = 2 × (12 + 10) × 8 = 352 sq ft.
- Openings = 1 doors × 21 sq ft + 2 windows × 15 sq ft = 51 sq ft, leaving 301 sq ft.
- Wall paint = 301 sq ft × 2 coats ÷ 350 sq ft per gallon = 1.72 gallons.
- Rounded up to quarts (4 to a gallon): 2 gallons. Three quarts cost about as much as a gallon, so three become a gallon.
Method and assumptions
The calculator finds the area of the four walls (the room’s perimeter × the wall height), subtracts 21 sq ft for each door (3 × 7 ft) and 15 sq ft for each window (3 × 5 ft), multiplies by the number of coats and divides by the coverage on the can.
Paint is sold in gallons and quarts, so the answer is rounded up to whole quarts, and three quarts become a gallon because they usually cost about the same.
Coverage is the biggest unknown. Manufacturers typically quote 350 to 400 sq ft per gallon per coat on smooth, primed walls. Bare drywall, textured walls, masonry and big color changes take more paint, so lower the coverage figure for those.
Tips
- Bare drywall needs a coat of primer first; work it out as a separate coat with the primer’s coverage.
- Keep a little paint for touch-ups; buying the last quart from the same batch keeps the color consistent.
- Trim, doors and cabinets usually take a different sheen. A quart covers the trim in most rooms.
Questions
How much paint do I need for a 12 × 12 room?
With 8 ft walls, a 12 × 12 room has 384 sq ft of wall. Less one door and two windows (51 sq ft) that is 333 sq ft. Two coats at 350 sq ft per gallon need 1.9 gallons: buy 2 gallons.
How many square feet does a gallon of paint cover?
Typically 350 to 400 sq ft for one coat on a smooth, primed surface. Rough, porous or unprimed surfaces can drop that to 200 to 300 sq ft.
Sources
Method and figures checked on . Every worked answer on this page is covered by an automated test. Found a mistake? Tell us.