Your 'EFC' is your Expected Family Contribution. It's a number that represents just how much the federal government thinks your family can contribute toward your teen's college expenses...every year! It is not only used by the federal government to determine if your family qualifies for need-based aid (the Pell Grant, for example) but colleges also use the EFC in determining need-based aid (and sometimes scholarships). However, most private colleges also use another set of calculations, called the CSS Profile.
But here it is. How the government calculates your EFC.