Carlisle is the smaller principal city of the Harrisburg–Carlisle Metropolitan Statistical Area, which includes all of Cumberland, Dauphin, and Perry counties.
As of the census of 2000, there were 17,970 people, 7,426 households, and 4,010 families residing in the borough. The population density was 3,308.9 people per square mile (1,277.8/km²). There were 8,032 housing units at an average density of 1,479.0/sq mi (571.1/km²). The racial makeup of the borough was 88.93% White, 6.92% African American, 0.14% Native American, 1.60% Asian, 0.02% Pacific Islander, 0.71% from other races, and 1.69% from two or more races. Hispanic or Latino of any race were 1.96% of the population.
There were 7,426 households out of which 23.3% had children under the age of 18 living with them, 40.3% were married couples living together, 10.9% had a female householder with no husband present, and 46.0% were non-families. 39.3% of all households were made up of individuals and 14.4% had someone living alone who was 65 years of age or older. The average household size was 2.10 and the average family size was 2.81.
In the borough the population was spread out with 18.6% under the age of 18, 17.2% from 18 to 24, 25.3% from 25 to 44, 21.1% from 45 to 64, and 17.8% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 84.2 males. For every 100 females age 18 and over, there were 80.8 males.
The median income for a household in the borough was $33,969, and the median income for a family was $46,588. Males had a median income of $34,519 versus $25,646 for females. The per capita income for the borough was $21,394. About 8.6% of families and 14.0% of the population were below the poverty line, including 21.7% of those under age 18 and 8.5% of those age 65 or over.