This county is a part of the Washington-Baltimore Metropolitan Area.
According to the U.S. Census Bureau, the county has a total area of 902 km² (348 mi²). 875 km² (338 mi²) of it is land and 27 km² (11 mi²) of it is water. The total area is 3.04% water.
As of the census2 of 2000, there are 280,813 people, 94,570 households, and 72,724 families residing in the county. The population density is 321/km² (831/mi²). There are 98,052 housing units at an average density of 112/km² (290/mi²). The racial makeup of the county is 68.93% White, 18.76% Black or African American, 0.39% Native American, 3.81% Asian, 0.13% Pacific Islander, 4.35% from other races, and 3.62% from two or more races. 9.74% of the population are Hispanic or Latino of any race.
There are 94,570 households out of which 44.20% have children under the age of 18 living with them, 61.30% are married couples living together, 11.20% have a female householder with no husband present, and 23.10% are non-families. 17.10% of all households are made up of individuals and 3.00% have someone living alone who is 65 years of age or older. The average household size is 2.94 and the average family size is 3.32.
In the county, the population is spread out with 30.40% under the age of 18, 8.80% from 18 to 24, 35.20% from 25 to 44, 20.80% from 45 to 64, and 4.80% who are 65 years of age or older. The median age is 32 years. For every 100 females there are 99.50 males. For every 100 females age 18 and over, there are 97.40 males.
The median income for a household in the county is $65,960, and the median income for a family is $71,622. Males have a median income of $45,595 versus $34,286 for females. The per capita income for the county is $25,641. 4.40% of the population and 3.30% of families are below the poverty line. Out of the total people living in poverty, 5.60% are under the age of 18 and 4.70% are 65 or older.