This database includes median family income estimates (MFI's) established by HUD.  See Metropolitan Statistical Areas (MSAs) for area definitions. Counties are generally aggregated into metropolitan areas and data reported reflect metropolitan area median income, not that for a member county. For example, in the case of California, income data reported for San Mateo County reflects San Francisco metropolitan area (San Francisco, Marin, and San Mateo counties) median income, not that for an individual county.

Median income estimates are based on decennial Census data, Census Current Population Surveys (CPS), American Community Surveys (ACS) for changes to state-level income, and Bureau of Labor Statistics (BLS) data for county-level changes to wages.

HUD is requred by law to set income limits that determine the eligibility of applicants for HUD's assisted housing programs and uses the Section 8 program's Fair Market Rent (FMR) area definitions. In addition to median incomes, low income estimates are divided into three categories:

• Low-income families, defined as families whose incomes do not exceed 80 percent of the MFI for the area.
• Very low-income families, defined as families whose incomes do not exceed 50 percent of the MFI for the area.
• Program targeting standard according to the 1998 Act amendments, or 30 percent of the MFI for the area.

Additional information: 

• 2003 Median Family Income Estimates were re-benchmarked with 2000 Census data, resulting in an unusual number of increases and decreases in the figures presented.
• Income limits are adjusted for areas with unusually high or low family income or housing-cost-to-income relationships.
• Income limits are adjusted for family size.