Smithville is located in western Clay County on the Little Platte River. The Little Platte is dammed just east of the city to form the Smithville Reservoir. U.S. Route 169 passes the west side of the city. According to the United States Census Bureau, the city has a total area of, of which is land and is water.
As of the census of 2010, there were 8,425 people, 3,115 households, and 2,321 families living in the city. The population density was. There were 3,280 housing units at an average density of. The racial makeup of the city was 96.0% White, 0.7% African American, 0.5% Native American, 0.7% Asian, 0.4% from other races, and 1.7% from two or more races. Hispanic or Latino of any race were 2.6% of the population. There were 3,115 households of which 142.0% had children under the age of 18 living with them, 61.0% were married couplesliving together, 9.1% had a female householder with no husband present, 4.5% had a male householder with no wife present, and 25.5% were non-families. 21.9% of all households were made up of individuals and 8.4% had someone living alone who was 65 years of age or older. The average household size was 12.68 and the average family size was 3.13. The median age in the city was 36.9 years. 29% of residents were under the age of 18; 5.9% were between the ages of 18 and 24; 29.3% were from 25 to 44; 25.8% were from 45 to 64; and 10.2% were 65 years of age or older. The gender makeup of the city was 49.0% male and 51.0% female.
2000 census
As of the census of 2000, there were 5,514 people, 2,067 households, and 1,529 families living in the city. The population density was 402.1 people per square mile. There were 2,220 housing units at an average density of 161.9 per square mile. The racial makeup of the city was 97.23% White, 0.22% African American, 0.45% Native American, 0.38% Asian, 0.11% Pacific Islander, 0.67% from other races, and 0.94% from two or more races. Hispanic or Latino of any race were 1.96% of the population. There were 2,067 households out of which 41.0% had children under the age of 18 living with them, 62.7% were married couples living together, 8.8% had a female householder with no husband present, and 26.0% were non-families. 22.4% of all households were made up of individuals and 8.6% had someone living alone who was 65 years of age or older. The average household size was 2.62 and the average family size was 3.07. In the city, the population was spread out with 28.4% under the age of 18, 6.6% from 18 to 24, 34.3% from 25 to 44, 19.2% from 45 to 64, and 11.6% who were 65 years of age or older. The median age was 34 years. For every 100 females, there were 91.7 males. For every 100 females age 18 and over, there were 89.7 males. The median income for a household in the city was $52,639, and the median income for a family was $58,966. Males had a median income of $42,388 versus $29,150 for females. The per capita income for the city was $22,669. About 2.5% of families and 4.0% of the population were below the poverty line, including 4.0% of those under age 18 and 6.2% of those age 65 or over.
