According to the U.S. Census Bureau, the county has a total area of 1,746 km² (674 mi²). 1,743 km² (673 mi²) of it is land and 4 km² (1 mi²) of it is water. The total area is 0.21% water.
As of the census2 of 2000, there are 39,825 people, 15,683 households, and 10,240 families residing in the county. The population density is 23/km² (59/mi²). There are 17,501 housing units at an average density of 10/km² (26/mi²). The racial makeup of the county is 93.24% White, 1.50% Black or African American, 0.59% Native American, 2.35% Asian, 0.06% Pacific Islander, 0.47% from other races, and 1.79% from two or more races. 1.22% of the population are Hispanic or Latino of any race.
There are 15,683 households out of which 30.30% have children under the age of 18 living with them, 52.70% are married couples living together, 9.50% have a female householder with no husband present, and 34.70% are non-families. 28.60% of all households are made up of individuals and 10.70% have someone living alone who is 65 years of age or older. The average household size is 2.38 and the average family size is 2.92.
In the county the population is spread out with 23.70% under the age of 18, 14.50% from 18 to 24, 26.10% from 25 to 44, 21.80% from 45 to 64, and 13.90% who are 65 years of age or older. The median age is 35 years. For every 100 females there are 103.20 males. For every 100 females age 18 and over, there are 102.90 males.
The median income for a household in the county is $29,378, and the median income for a family is $38,693. Males have a median income of $29,428 versus $19,893 for females. The per capita income for the county is $16,084. 16.40% of the population and 11.30% of families are below the poverty line. Out of the total people living in poverty, 19.00% are under the age of 18 and 13.00% are 65 or older.