There are two major definitions of “rural” that the Federal government uses, along with many variants that are also available.
The Census does not actually define “rural.” “Rural” encompasses all population, housing, and territory not included within an urban area. Whatever is not urban is considered rural.
The Census recognizes that "densely settled communities outside the boundaries of large incorporated municipalities were just as ''urban'' as the densely settled population inside those boundaries." Their definition does not follow city or county boundaries and so it is difficult sometimes to determine whether a particular area is considered urban or rural. Under this definition, about 21% of the US population in 2000 was considered rural but over 95% of the land area was classified as rural. In the 2010 Census 59.5 million people, 19.3% of the population, was rural while over 95% of the land area is still classified as rural.
The White House Office of Management and Budget (OMB) designates counties as Metropolitan, Micropolitan, or Neither. A Metro area contains a core urban area of 50,000 or more population, and a Micro area contains an urban core of at least 10,000 (but less than 50,000) population. All counties that are not part of a Metropolitan Statistical Area (MSA) are considered rural. Micropolitan counties are considered non-Metropolitan or rural along with all counties that are not classified as either Metro or Micro. Under this definition about 17% of the population in 2000 was considered Non-Metro while 74% of the land area was contained in Non-Metro counties. After the 2010 Census, the Non-Metro population was 46.2 million people, about 15% of the total population. For more information on Metro areas, see: United States Census Bureau Metropolitan and Micropolitan Statistical Areas Main.
There are measurement challenges with both the Census and OMB definitions. Some policy experts note that the Census definition classifies quite a bit of suburban area as rural. The OMB definition includes rural areas in Metropolitan counties including, for example, the Grand Canyon which is located in a Metro county. Consequently, one could argue that the Census Bureau standard includes an overcount of rural population whereas the OMB standard represents an undercount of the rural population.
The Office of Rural Health Policy (ORHP) accepts all non-metro counties as rural and uses an additional method of determining rurality called the Rural-Urban Commuting Area (RUCA) codes. Like the MSAs, these are based on Census data which is used to assign a code to each Census Tract. Tracts inside Metropolitan counties with the codes 4-10 are considered rural. While use of the RUCA codes has allowed identification of rural census tracts in Metropolitan counties, among the more than 70,000 tracts in the U.S. there are some that are extremely large and where use of RUCA codes alone fails to account for distance to services and sparse population. In response to these concerns, ORHP has designated 132 large area census tracts with RUCA codes 2 or 3 as rural. These tracts are at least 400 square miles in area with a population density of no more than 35 people. Following the 2010 Census the ORHP definition included approximately 57 million people, about 18% of the population and 84% of the area of the USA. RUCA codes represent the current version of the Goldsmith Modification.
For more information on RUCAs, see:
The HRSA website has a Rural Health Grants Eligibility Analyzer webpage where you can search for eligible counties, or eligible census tracts inside Metro counties, at Rural Health Grants Eligibility Analyzer. You can also download a complete list of eligible areas from that page.