As of the census of 2000, there were 15,121 people, 6,740 households, and 3,626 families residing in the city. The population density was 5,611.3 people per square mile (2,170.4/km²). There were 7,402 housing units at an average density of 2,746.8/sq mi (1,062.4/km²). The racial makeup of the city was 95.52% White, 2.22% African American, 0.14% Native American, 0.44% Asian, 0.03% Pacific Islander, 0.52% from other races, and 1.14% from two or more races. Hispanic or Latino of any race were 1.42% of the population.
There were 6,740 households out of which 26.8% had children under the age of 18 living with them, 35.0% were married couples living together, 14.4% had a female householder with no husband present, and 46.2% were non-families. 40.7% of all households were made up of individuals and 16.3% had someone living alone who was 65 years of age or older. The average household size was 2.18 and the average family size was 2.96.
In the city the population was spread out with 23.7% under the age of 18, 9.7% from 18 to 24, 30.3% from 25 to 44, 20.2% from 45 to 64, and 16.1% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 88.1 males. For every 100 females age 18 and over, there were 83.9 males.
The median income for a household in the city was $25,154, and the median income for a family was $35,893. Males had a median income of $30,607 versus $20,950 for females. The per capita income for the city was $16,457. About 14.7% of families and 19.1% of the population were below the poverty line, including 26.8% of those under age 18 and 14.5% of those age 65 or over.
There are two airports located outside the city. Butler County Airport is used for general aviation, and can accommodate large aircraft such as corporate jets. Butler Farm Show Airport is used by pilots with smaller, private aircraft in the Butler area.
Butler is serviced by the The Bus which is run by the Butler Transit Authority.
There are currently two railroads that have service in Butler. Both railways are freight haulers. The Canadian National Railway's (formerly the Bessemer and Lake Erie Railroad) main line passes through the city, while the Buffalo and Pittsburgh Railroad provides regional service in the area. The B&P has a large locomotive shop located just outside the city limits.
Five major highways run through or near the city, providing links to other areas throughout Western Pennsylvania. The south terminus of PA 38 terminates just north of the city at U.S. Route 422. U.S. Route 422 skirts the city to the north on the Butler Bypass.PA 68, and PA 356 go straight through downtown where they intersect with PA 8. PA 8 is Butler's Main Street when passing through the city.