How do I determine the amount of paint needed for a room?
A can of paint will often indicate how much area it will cover. A general rule of thumb is 350 sq ft per gallon (as Shirlock mentions, use 250-300 sqft/gal for the first coat). Calculating the area of the walls and ceiling involves some basic math: multiply the length * width (or height) of the ceiling (or wall), then subtract any openings.