Ada County is part of the Boise City-Nampa, Idaho Metropolitan Statistical Area.
As of the census of 2000, there were 300,904 people, 113,408 households, and 77,344 families residing in the county. The population density was 285/mi² (110/km²). There were 118,516 housing units at an average density of 112/mi² (43/km²). The racial makeup of the county was 92.86% White, 0.65% Black or African American, 0.69% Native American, 1.74% Asian, 0.15% Pacific Islander, 1.67% from other races, and 2.24% from two or more races. Hispanic or Latino of any race were 4.48% of the population.
There were 113,408 households out of which 36.20% had children under the age of 18 living with them, 55.10% were married couples living together, 9.40% had a female householder with no husband present, and 31.80% were non-families. 23.80% of all households were made up of individuals and 6.07% had someone living alone who was 65 years of age or older. The average household size was 2.59 and the average family size was 3.11.
In the county the population was spread out with 27.30% under the age of 18, 10.30% from 18 to 24, 32.50% from 25 to 44, 20.80% from 45 to 64, and 9.10% who were 65 years of age or older. The median age was 33 years. For every 100 females there were 100.6 males. For every 100 females age 18 and over, there were 98.9 males.
The median income for a household in the county was $46,140, and the median income for a family was $54,416. Males had a median income of $37,867 versus $26,453 for females. The per capita income for the county was $22,519. About 5.40% of families and 7.70% of the population were below the poverty line, including 9.20% of those under age 18 and 5.70% of those age 65 or over.