The short answer
A 12 × 12 ft ceiling is 144 sq ft. Two coats at 350 sq ft per gallon need 0.82 gallon, so buy 1 gallon, about $45. A 20 × 12 ft vaulted ceiling with a 4 ft rise is 288 sq ft, not 240.
The vaulted formula
For a 20 ft long, 12 ft wide ceiling with a 4 ft rise, each slope is √(36 + 16) = 7.21 ft wide, so the area is 2 × 7.21 × 20 = 288 sq ft versus 240 flat. This assumes a simple two-sided cathedral (a ridge down the middle). Odd shapes: use the square footage calculator and enter the area directly.
FAQ
How much paint do I need for a ceiling?
Length × width gives the area. Two coats at 350 sq ft per gallon is area × 2 ÷ 350. A 12 × 12 ft ceiling needs 0.82 gallon, so one gallon covers it.
How do I measure a vaulted ceiling?
Measure the room length, the width across the walls and the rise from the wall tops to the peak. The calculator finds the sloped surface for you.
Is ceiling paint different?
Ceiling paint is usually a flat finish. Use the coverage on the can; 350 is our default.