Butler County's population was estimated to be 63,147 in the year 2006, an increase of 3,450, or +5.8%, over the previous six years; it has the seventh fastest growing and eighth largest population in the state.
As of the U.S. Census in 2000, there were 59,482 people, 21,527 households, and 16,059 families residing in the county. The population density was 42 people per square mile (16/km²). There were 23,176 housing units at an average density of 16 per square mile (6/km²). The racial makeup of the county was 94.94% White, 1.38% Black or African American, 0.91% Native American, 0.40% Asian, 0.03% Pacific Islander, 0.66% from other races, and 1.69% two or more races. Hispanic or Latino of any race were 2.25% of the population.
There were 21,527 households out of which 37.90% had children under the age of 18 living with them, 62.60% were married couples living together, 8.30% had a female householder with no husband present, and 25.40% were non-families. 21.90% of all households were made up of individuals and 9.40% had someone living alone who was 65 years of age or older. The average household size was 2.67 and the average family size was 3.13.
In the county the population was spread out with 28.60% under the age of 18, 8.30% from 18 to 24, 28.80% from 25 to 44, 21.70% from 45 to 64, and 12.60% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 100.90 males. For every 100 females age 18 and over, there were 98.80 males.
The median income for a household in the county was $45,474, and the median income for a family was $53,632. Males had a median income of $38,675 versus $26,109 for females. The per capita income for the county was $20,150. About 5.40% of families and 7.30% of the population were below the poverty line, including 9.00% of those under age 18 and 6.40% of those age 65 or over.