It's a definition in the census. I find it somewhat disappointing to see such a dismissive response when the answer is so easy to find.

Here's a link to the definitions of urban and rural: https://www.census.gov/geo/reference/gtc/gtc_urbanrural.html

Here's the 2010 figures for the split between the urban and rural: https://www.census.gov/geo/reference/ua/urban-rural-2010.htm...