The poverty line is defined by the federal government. It changes year to year and gradually goes up over time. The poverty line depends on the number of people in the family. It is the responsibility of the Dept. of Health and Human Services to publish the poverty rate. I found the 2011 figures at their website.
The numbers are :
|Persons in family||Poverty guideline|
|For families with more than 8 persons, add $3,820 for each additional person.|
The poverty line is a bit more for Alaska and Hawaii.