As of 2000, the population was 597,635. A 2004 U.S. Census estimate placed the population at 621,342, making it the fourth most populous county in Pennsylvania (after Philadelphia, Allegheny, and Montgomery counties), and the 95th most populous county in the United States. As of 2000, it is the 76th wealthiest county in the nation as measured by median family income.
As of the census of 2000, there were 597,635 people, 218,725 households, and 160,981 families residing in the county. The population density was 984 people per square mile (380/km²). There were 225,498 housing units at an average density of 371 per square mile (143/km²). The racial makeup of the county was 90.01% White, 4.08% Black or African American, 0.14% Native American, 3.10% Asian, 0.04% Pacific Islander, 0.90% from other races, and 1.09% from two or more races. 3.16% of the population were Hispanic or Latino of any race. 20.1% were of German, 19.1% Irish, 14.0% Italian, 7.5% English and 5.9% Polish ancestry according to Census 2000.
There were 218,725 households out of which 35.30% had children under the age of 18 living with them, 61.20% were married couples living together, 8.80% had a female householder with no husband present, and 26.40% were non-families. 21.50% of all households were made up of individuals and 8.10% had someone living alone who was 65 years of age or older. The average household size was 2.69 and the average family size was 3.17.
In the county, the population was spread out with 25.70% under the age of 18, 7.00% from 18 to 24, 30.70% from 25 to 44, 24.30% from 45 to 64, and 12.40% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 96.30 males. For every 100 females age 18 and over, there were 93.40 males.
The median income for a household in the county was $59,727, and the median income for a family was $68,727 (these figures had risen to $71,161 and $86,493 respectively as of a 2007 estimate). Males had a median income of $46,587 versus $31,984 for females. The per capita income for the county was $27,430. About 3.10% of families and 4.50% of the population were below the poverty line, including 4.80% of those under age 18 and 5.50% of those age 65 or over.
Like the rest of the Philadelphia region, Bucks County is experiencing a rapid increase of immigrants since the 2000 census. A 2005 population estimate of Bucks, showed that the Indian and Mexican populations have already doubled since 2000. Koreans, Palestinians, Turks, and Russians also saw significant increases.