According to US Census Bureau, middle class is defined as families with income in middle three income quintiles. In 2010, income level for each quin tile is:

Quin tile Lowest Income Share of US Household Income
5th $99,891 50.3%
4th $62,150 23.1%
3rd $39,211 14.6%
2rd $20,699 8.5%
1rd 3.3%

If you household income was between $20,699 to $99,891, you belong to United States middle class.