It varies by the size of the family and the size of the community. I'll give you an example. I'll use 2004, because the data I used were from 2004, so it's two years behind.
Say you are looking at one mother with one child in a large city with a population of 500,000. If her before-tax earnings were $25,319, she would be considered living below the low-income cut-off, which is the de facto poverty line that's been in use for about fifty years.