A block group (BG) is a cluster of census blocks having the same first digit of their four-digit identifying numbers within a census tract. For example, block group 3 (BG 3) within a census tract includes all blocks numbered from 3000 to 3999. BGs generally contain between 600 and 3,000 people, with an optimum size of 1,500 people.

BGs are the lowest level of the geographic hierarchy for which the U.S. Census Bureau tabulates and presents sample data

BGs on American Indian reservations, offreservation trust lands, and special places must contain a minimum of 300 people. (Special places include correctional institutions, military installations, college campuses, worker’s dormitories, hospitals, nursing homes, and group homes.)

In decennial census data tabulations, a BG may be split to present data for every unique combination of American Indian area, Alaska Native area, Hawaiian home land, congressional district, county subdivision, place, voting district, or other tabulation entity shown in the data products.

For example, if BG 3 is partly in a city and partly outside the city, there are separate tabulated records for each portion of BG 3. BGs are used in tabulating data nationwide, as was done for the
1990 census, for all block-numbered areas in the 1980 census, and for selected areas in the 1970 census.

For data presentation purposes, BGs are a substitute for the enumeration districts (EDs) used for reporting data in many parts of the United States for the 1970 and 1980 censuses and in all areas before 1970. 

All territory in the United States, Puerto Rico, and the Island Areas has been assigned block numbers, as was the case for the 1990 census.

To improve operational efficiency and geographic identifications, the U.S. Census Bureau has introduced different numbering systems for tabulation blocks used in decennial census data products, and for collection blocks, used in administering the census. (In 1990, there generally was a single numbering system.)

Tabulation blocks do not cross the boundaries of any entity for which the U.S. Census Bureau tabulates data, including American Indian areas, Alaska Native areas, Hawaiian home lands, census tracts, congressional districts, counties, county subdivisions, places, state legislative districts, urban and rural areas, school districts, voting districts, and ZIP Code® tabulation areas. Tabulation blocks also generally do not cross the boundaries of certain landmarks, including military installations, national parks, and national monuments.

Tabulation blocks are identified uniquely within census tract by means of a four-digit number. (The 1990 census block numbers had three digits, with a potential alphabetic suffix.) The Census 2000 collection blocks are numbered uniquely within county (or statistically equivalent entity), and consist of four or five digits. For its Census 2000 data tabulations, the U.S. Census Bureau created a unique set of census block numbers immediately before beginning the tabulation process. These are the census block numbers seen in the data presentations.

For the 1990 census, the U.S. Census Bureau created a separate block with a suffix of ‘‘Z’’ to identify crews-of-vessels population. For Census 2000, crews-of-vessels population is assigned to the land block identified by the U.S. Census Bureau as associated with the home port of the vessel.