The United States Office of Management and Budget (OMB) delineates metropolitan and micropolitan statistical areas according to published standards that are applied to Census Bureau data. The general concept of a metropolitan or micropolitan statistical area is that of a core area containing a substantial population nucleus, together with adjacent communities having a high degree of economic and social integration with that core. Currently delineated metropolitan and micropolitan statistical areas are based on application of 2020 standards (which appeared in the Federal Register on July 16, 2021) to 2020 Census and 2016-2020 American Community Survey data, as well as Vintage 2021 Population Estimates Program data. Current metropolitan and micropolitan statistical area delineations were announced by OMB effective July 2023.

Standard delineations of metropolitan areas were first issued in 1949 by the then Bureau of the Budget (predecessor of OMB), under the designation "standard metropolitan area" (SMA). The term was changed to "standard metropolitan statistical area" (SMSA) in 1959, and to "metropolitan statistical area" (MSA) in 1983. The term "metropolitan area" (MA) was adopted in 1990 and referred collectively to metropolitan statistical areas (MSAs), consolidated metropolitan statistical areas (CMSAs), and primary metropolitan statistical areas (PMSAs). The term "core based statistical area" (CBSA) became effective in 2000 and refers collectively to metropolitan and micropolitan statistical areas.

OMB has been responsible for the official metropolitan areas since they were first delineated, except for the period 1977 to 1981, when they were the responsibility of the Office of Federal Statistical Policy and Standards, Department of Commerce. The standards for delineating metropolitan areas were modified in 1958, 1971, 1975, 1980, 1990, 2000, 2010, and 2021.

2020 Standards for Delineating Core Based Statistical Areas

Delineating Metropolitan and Micropolitan Statistical Areas

The 2020 standards provide that each CBSA must contain at least one urban area of 10,000 or more population. Each metropolitan statistical area must have at least one urban area of 50,000 or more inhabitants. Each micropolitan statistical area must have at least one urban area of at least 10,000 but less than 50,000 population.

Under the standards, the county (or counties) in which at least 50 percent of the population resides within urban areas of 10,000 or more population, or that contain at least 5,000 people residing within a single urban area of 10,000 or more population, is identified as a "central county" (counties). Additional "outlying counties" are included in the CBSA if they meet specified requirements of commuting to or from the central counties. Counties or equivalent entities form the geographic "building blocks" for metropolitan and micropolitan statistical areas throughout the United States and Puerto Rico.

If specified criteria are met, a metropolitan statistical area containing a single core with a population of 2.5 million or more may be subdivided to form smaller groupings of counties referred to as "metropolitan divisions."

As of July 2023, there are 387 metropolitan statistical areas and 538 micropolitan statistical areas in the United States.

Principal Cities and Metropolitan and Micropolitan Statistical Area Titles

The largest city in each metropolitan or micropolitan statistical area is designated a "principal city." Additional cities qualify if specified requirements are met concerning population size and employment. The title of each metropolitan or micropolitan statistical area consists of the names of up to three of its principal cities and the name of each state into which the metropolitan or micropolitan statistical area extends. Titles of metropolitan divisions also typically are based on principal city names but in certain cases consist of county names.

Changes in Delineations over Time

Changes in the delineations of these statistical areas since the 1950 census have consisted chiefly of:

• the recognition of new areas as they reached the minimum required urban area or city population, and
• the addition of counties to existing areas as new commuting and urban area data showed them to qualify.

In some instances, formerly separate areas have been merged, components of an area have been transferred from one area to another, or components have been dropped from an area. The large majority of changes have taken place on the basis of decennial census (and more recently American Community Survey) data. However, Census Bureau Population Estimates Program and American Community Survey data serve as the basis for intercensal updates in specified circumstances.

Because of these historical changes in geographic delineations, users must be cautious in comparing data for these statistical areas from different dates. For some purposes, comparisons of data for areas as delineated at given dates may be appropriate; for other purposes, it may be preferable to maintain consistent area delineations. Historical statistical area delineations are available for selected years from 1950 to 2020.

Uses of Metropolitan and Micropolitan Statistical Areas

While OMB recognizes that a number of agencies, both inside and outside the Federal government, make use of the delineations of metropolitan and micropolitan statistical areas for nonstatistical programmatic applications, OMB delineates the areas for statistical purposes only. In delineating metropolitan and micropolitan statistical areas, OMB does not attempt to anticipate or take into account any nonstatistical uses that may be made of the delineations, nor will OMB modify the delineations to meet the requirements of any nonstatistical program. Questions about how metropolitan or micropolitan statistical areas are used within any particular nonstatistical program should be directed to the agency that administers that program.