DeKalb County is included in the Atlanta-Sandy Springs-Marietta, Georgia, metropolitan statistical area. It is the third-most-populated county in the Atlanta area and the state, just behind Gwinnett County. DeKalb is primarily a suburban county, and is the second-most-affluent county with an African-American majority in the United States, behind Prince George's County, Maryland.
As of the census of 2000, there were 665,865 people, 249,339 households, and 156,584 families residing in the county. The population density was 2,483 people per square mile (959/km²). There were 261,231 housing units at an average density of 974 per square mile (376/km²). The racial makeup of the county was 54.23% Black or African American, 35.82% White, 0.23% Native American, 4.01% Asian, 0.05% Pacific Islander, 3.53% from other races, and 2.12% from two or more races. 7.89% of the population were Hispanic or Latino of any race.
There were 249,339 households out of which 31.00% had children under the age of 18 living with them, 40.10% were married couples living together, 17.60% had a female householder with no husband present, and 37.20% were non-families. 26.90% of all households were made up of individuals and 5.20% had someone living alone who was 65 years of age or older. The average household size was 2.62 and the average family size was 3.20.
In the county the population was spread out with 24.60% under the age of 18, 10.90% from 18 to 24, 36.70% from 25 to 44, 19.70% from 45 to 64, and 8.00% who were 65 years of age or older. The median age was 32 years. For every 100 females there were 94.10 males. For every 100 females age 18 and over, there were 90.80 males.
The median income for a household in the county was $49,117, and the median income for a family was $54,018. Males had a median income of $36,270 versus $31,653 for females. The per capita income for the county was $23,968. About 7.80% of families and 10.80% of the population were below the poverty line, including 14.10% of those under age 18 and 8.70% of those age 65 or over.
Although Fulton County is more populous, DeKalb has the highest population density of any county in the Atlanta metropolitan area.