Areas Available for Analysis: nation, regions, states, counties, census tracts, block groups
school districts, congressional districts, places, metropolitan areas
Census Tracts: subdivisions of a county that are relatively small areas with a population of about 1,200 to 8,000 and about 480 to 3,200 housing units. Data at the Census Tract level is only available in 5-year estimates.
Block Groups: A subdivision of Census Tracts that are the lowest level of geography for the ACS and they have a population of about 600 to 3,000 and 240 to 1,200 housing units. Data at the Block Group level is only available in 5-year estimates.
Blocks: Only the Census is available at the Block level, and so if you want data at the Block level you'll have to find Census data. The Census Blocks usually follow city blocks within the city.
Geographies provide a framework and context for our research. It is important to understand the geography of the ACS. The population of the area you're interested in will determine whether you can find 1-year, 3-year or 5-year estimates.
1-year estimate: Areas with a population of 65,000+ are available in 1-year estimates.
3-year estimate: Areas with a population of 20,000+ are available in 3-year estimates (ex: 2007-2009 3-year estimate includes data collected between January 1, 2007 and December 31, 2009). A 3-year estimate has 36 months worth of data. If you compare 3-year estimates, you should select year ranges that do not overlap.
5-year estimate: All areas, regardless of population size, are available in 5-year estimates (ex: 2005-2009, 5-year estimate includes data collected between January 1, 2005 and December 31, 2009). A 5-year estimate has 60 months worth of data. If you compare 5-year estimates, you should select year ranges that do not overlap.