Kentucky environmental lawyer Sanders says there are websites to help you estimate proper amount of paint to purchase without eventually discarding left over paint.

An EPA study issued in April 2007 estimates that about 10 percent of the U.S. house paint purchased each year – about 65 to 69 million gallons – is ultimately discarded.  To help you buy just enough, Earth 911 —  http://www.earth911.org – has tips on its Paint Wise page and there are several paint calculators online.