The Census Bureau produces population estimates for the nation, the states, the District of Columbia, Puerto Rico, counties and equivalents (including municipios in Puerto Rico), incorporated places, minor civil divisions, consolidated cities, census regions and divisions, and core-based statistical areas (metropolitan and micropolitan statistical areas).

The Census Bureau delineates two sets of sub-national areas that are composed of states. This two-tiered system of areas consists of nine census divisions nested in four census regions. The Northeast region includes the New England division: Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont; and the Middle Atlantic division: New Jersey, New York, and Pennsylvania. The Midwest region includes the East North Central division: Illinois, Indiana, Michigan, Ohio, and Wisconsin; and the West North Central division: Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, and South Dakota. The South region includes the South Atlantic division: Delaware, District of Columbia, Florida, Georgia, Maryland, North Carolina, South Carolina, Virginia, West Virginia; the East South Central division: Alabama, Kentucky, Mississippi, and Tennessee; and the West South Central division: Arkansas, Louisiana, Oklahoma, and Texas. The West region includes the Mountain division: Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, and Wyoming; and the Pacific division: Alaska, California, Hawaii, Oregon, and Washington.

Counties are the primary legal divisions of most states. Most counties are functioning governmental units, whose powers and functions vary from state to state. In Louisiana, these primary divisions are known as parishes. In Alaska, the county equivalents consist of legally organized boroughs, municipalities, and "census areas" delineated for statistical purposes by the State of Alaska and the Census Bureau (since 1980). In four states (Maryland, Missouri, Nevada, and Virginia), one or more cities are independent of any county organization and thus constitute primary divisions of their states; the Census Bureau refers to these places as "independent cities" and treats them as the equivalents of counties for statistical purposes. The District of Columbia has no primary divisions and the jurisdiction is treated as the equivalent of a county. In Puerto Rico, municipios are the primary divisions and treated as county equivalents for statistical purposes. Legal changes to county boundaries or names are typically infrequent, but do occur from time to time.

Legally defined county subdivisions are referred to as minor civil divisions (MCDs). MCDs are the primary divisions of a county. They comprise both governmentally functioning entities — that is, those with elected or appointed officials who provide services and raise revenues — and nonfunctioning entities that exist primarily for administrative purposes, such as election districts. Twenty-eight states and Puerto Rico have MCDs. However, the MCDs function as general purpose governmental units in all or part of only twenty states. Within these twenty states, the Census Bureau produces estimates for all governmentally functioning MCDs and for nonfunctioning MCDs in counties that contain at least one functioning MCD.

The legal powers and functions of MCDs vary from state to state. Most of the MCDs in twelve states (Connecticut, Maine, Massachusetts, Michigan, Minnesota, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont, and Wisconsin) serve as general-purpose local governments. In the remaining eight states where the Census Bureau produces MCD level estimates (Illinois, Indiana, Kansas, Missouri, Nebraska, North Dakota, Ohio, and South Dakota) the MCDs, for the most part, perform less of a governmental role and are less well known locally, even though they are active governmental units.

MCDs are commonly known as towns (in New England, New York, and Wisconsin), townships, and districts, but also include a variety of other lesser known identifiers. In Maine and New York, American Indian reservations are not part of any other MCD and therefore, the Census Bureau treats them as MCD equivalents. The Population Estimates Program (PEP) does not produce separate estimates for American Indian Reservations regardless of their MCD status. In some states, all or some incorporated places are subordinate to the MCDs in which they are located. Therefore, a place may be either independent of or dependent within MCDs. In one state (Ohio), a multi-county place may be treated differently from county to county. No general purpose functioning MCDs exist in Puerto Rico.

The legal designations, powers, and functions of incorporated places vary from state to state. Incorporated places include cities, towns (except in New England, New York, and Wisconsin where the Census Bureau recognizes towns as MCDs for census purposes), boroughs (except in Alaska, where the Census Bureau recognizes boroughs as equivalents of counties, and New York, where the Census Bureau recognizes the five boroughs that constitute New York City as MCDs), villages, and other lesser known identifiers. Incorporated places can cross both county and MCD boundaries. In such cases a separate record is shown in the population estimates for the part of a place in each of its parent counties or MCDs. For such records, the place name is followed by the designation "pt" (which stands for part). The PEP produces estimates of the unincorporated "balance of county" area for counties that are not entirely composed of incorporated places. Another way to understand this is to think of the "balance of county" as the county population minus the county population resident within incorporated places.

Consolidated cities are a unit of government for which the functions of an incorporated place and its parent county or MCD have merged. The legal aspects of this action may result in both the primary incorporated place and the county or MCD continuing to exist as legal entities, even though the county or MCD performs few or no governmental functions. Where one or more other incorporated places within the consolidated government continue to function as separate governmental units, the primary incorporated place is referred to as a "consolidated city."

Estimates will be shown for consolidated cities and the consolidated city "balance," which is the consolidated city minus the semi-independent incorporated places located within the consolidated city. Consolidated cities include: Athens-Clark County, GA; Augusta-Richmond County, GA; Butte-Silver Bow, MT; Greeley County, KS; Indianapolis, IN; Louisville/Jefferson County, KY; Milford, CT; and Nashville-Davidson, TN. Estimates also are produced for the semi-independent places which together with the "balance record," sum to the entire territory of the consolidated city.

Geographic summary levels are three-digit numeric codes used by the Census Bureau to designate different geographic levels or areas. The following summary level codes are used in the presentation of the subcounty population estimates:

• 010: Nation
• 020: Region - Four groupings of states established by the Census Bureau in 1942 for the presentation of census data.
• 030: Division - A grouping of States within a census geographic region, established by the Census Bureau for the presentation of census data.
• 040: State - the primary legal subdivision of the United States
• 050: State-County - the primary legal subdivision of every state except Alaska and Louisiana
• 061: State-County-Minor Civil Division (MCD) - the Census Bureau produces estimates for MCDs in 20 states. These legally defined county subdivisions are known by various descriptions, including towns (in New England, New York and Wisconsin), townships, and districts.
• 071: State-County-MCD-incorporated place/balance - incorporated places can cross MCD boundaries, therefore a place name (with the "pt." flag following it) can appear in more than one MCD. The 071 code designates that part of a place that is within one MCD. This summary level includes any unincorporated "balance" of the MCD.
• 157: State-County-incorporated place/balance - incorporated places can cross county boundaries, therefore a place name (with the "pt." flag following it) can appear in more than one county. The 157 code designates that part of a place that is within one county or the unincorporated "balance" of the county.
• 162: State-Incorporated Place total
• 170: State-Consolidated City - An incorporated place that has combined its governmental functions with a county or subcounty entity but contains one or more incorporated places that continue to function as local governments within the consolidated government.
• 172: State-Consolidated City-Place within Consolidated City - An incorporated place or consolidated city "balance" within a consolidated city

These geographic areas comprise a smaller set of areas than those defined for all the Census Bureau's programs. More geographic information is available on the Census Bureau's Geography Division website. For a state-by-state summary of changes to the geography for Census 2010, go to Geographic Boundary Change Notes.