Franklin County is located in the U.S. state of Missouri. As of the 2010 census, the population was 101,492. Its county seat is Union. The county was organized in 1818 and is named after Founding Father Benjamin Franklin.
According to the U.S. Census Bureau, the county has a total area of 931 square miles (2,410 km), of which 923 square miles (2,390 km) is land and 8.0 square miles (21 km) (0.9%) is water. It is the fourth-largest county in Missouri by land area and third-largest by total area.
The center of the Missouri River forms the nominal northern border of the county, although the river has changed its course since boundaries were first established: a portion of St. Charles County near St. Albans is now south of the river, while a portion of Franklin County near Augusta is north of the river.
The Bourbeuse River flows for 107 miles through the county. It cuts a deep, narrow valley and is very crooked. It empties into the Meramec River near Union. This river is mostly undeveloped, with limited access and few bridges over it. During low water, a number of fords allow crossing.
The county is located in the Ozarks region, with steep hills and deep valleys, caves, springs, and sinkholes characteristic of karst areas. The underlying rock is typically carbonate, including limestone and dolomite. Mining activity in the county included ores of lead, copper, zinc, and deposits of refractory clay. The soils in most of the county tend to be thin, rocky red clay, and are poor for most agriculture, while the soil near the Missouri River is dark, rich, and thick, and used primarily for row crops such as corn, wheat, and soybeans. Much of the county is covered with thick forests, reestablished since the 1920s.
Urbanization is increasing in the county, especially surrounding Washington and Union, and along Interstate 44. St. Albans is now a continuation of the suburban region of St. Louis County while the majority of the county retains a rural character and includes extensive wilderness areas, typical of exurban areas.
As of the census of 2000, there were 93,807 people, 34,945 households, and 25,684 families residing in the county. The population density was 102 people per square mile (39/km²). There were 38,295 housing units at an average density of 42 per square mile (16/km²). The racial makeup of the county was 97.47% White, 0.94% Black or African American, 0.27% Asian, 0.24% Native American, 0.02% Pacific Islander, 0.20% from other races, and 0.86% from two or more races. Approximately 0.72% of the population were Hispanic or Latino of any race. 44.9% were of German, 13.0% American, 10.7% Irish and 7.7% English ancestry.
There were 34,945 households out of which 36.10% had children under the age of 18 living with them, 60.40% were married couples living together, 9.00% had a female householder with no husband present, and 26.50% were non-families. 22.10% of all households were made up of individuals and 8.90% had someone living alone who was 65 years of age or older. The average household size was 2.66 and the average family size was 3.11.
In the county, the population was spread out with 27.40% under the age of 18, 8.20% from 18 to 24, 30.00% from 25 to 44, 22.40% from 45 to 64, and 12.10% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 98.50 males. For every 100 females age 18 and over, there were 95.80 males.
The median income for a household in the county was $54,392, and the median income for a family was $62,969. Males had a median income of $35,849 versus $23,344 for females. The per capita income for the county was $24,529. About 4.50% of families and 7.00% of the population were below the poverty line, including 7.90% of those under age 18 and 8.80% of those age 65 or over.