Harker Heights is part of the Killeen–Temple–Fort Hood Metropolitan Statistical Area.
As of the census of 2000, there were 17,308 people, 6,200 households, and 4,712 families residing in the city. The population density was 1,356.4 people per square mile (523.7/km²). There were 6,823 housing units at an average density of 534.7/sq mi (206.5/km²). The racial makeup of the city was 70.89% White, 14.94% African American, 0.78% Native American, 3.58% Asian, 0.40% Pacific Islander, 5.79% from other races, and 3.62% from two or more races. Hispanic or Latino of any race were 12.44% of the population.
There were 6,200 households out of which 42.8% had children under the age of 18 living with them, 61.4% were married couples living together, 11.2% had a female householder with no husband present, and 24.0% were non-families. 18.0% of all households were made up of individuals and 3.5% had someone living alone who was 65 years of age or older. The average household size was 2.77 and the average family size was 3.13.
In the city the population was spread out with 30.4% under the age of 18, 10.5% from 18 to 24, 34.8% from 25 to 44, 18.0% from 45 to 64, and 6.2% who were 65 years of age or older. The median age was 30 years. For every 100 females there were 101.0 males. For every 100 females age 18 and over, there were 99.0 males.
The median income for a household in the city was $42,947, and the median income for a family was $49,607. Males had a median income of $31,728 versus $26,404 for females. The per capita income for the city was $20,061. About 7.4% of families and 10.0% of the population were below the poverty line, including 12.2% of those under age 18 and 4.9% of those age 65 or over.