|Elevation||686 ft (209 m)|
|Area||2.42 sq mi (6 km2)|
|- land||2.42 sq mi (6 km2)|
|- water||0.00 sq mi (0 km2)|
|Density||3,053.3 / sq mi (1,179 / km2)|
|- summer (DST)||CDT (UTC-5)|
|Area code||815 & 779|
|Wikimedia Commons: Somonauk, Illinois|
The DeKalb County portion of Somonauk is part of the Chicago-Naperville-Elgin, IL-IN-WI Metropolitan Statistical Area, while the small portion that lies in LaSalle County is part of the Ottawa–Peru, IL Micropolitan Statistical Area.
Somonauk is located at  The village is situated along the boundary between DeKalb and LaSalle counties. In the 2000 census, 1,190 of Somonauk's 1,295 residents (91.9%) lived in DeKalb County and 105 (8.1%) lived in LaSalle County.(41.6327194, -88.6865175).
According to the 2010 census, Somonauk has a total area of 2.415 square miles (6.25 km2), of which 2.41 square miles (6.24 km2) (or 99.79%) is land and 0.005 square miles (0.01 km2) (or 0.21%) is water.
Sannauk Forest Preserve serves as the unofficial boundary between Somonauk and Sandwich, which resides to the east. The forest preserve, owned and managed by DeKalb County, sits along Somonauk Creek, a tributary of the Fox River. The nearby Lake Holiday area includes a man-made lake created by the damming of Somonauk Creek. An old gravel pit was also was flooded and is part of the lake.
As of the census of 2000, there were 1,295 people, 500 households, and 347 families residing in the village. The population density was 2,113.4 people per square mile (819.7/km²). There were 512 housing units at an average density of 835.6 per square mile (324.1/km²). The racial makeup of the village was 97.45% White, 0.31% Native American, 0.15% Asian, 0.77% from other races, and 1.31% from two or more races. Hispanic or Latino of any race were 2.01% of the population.
There were 500 households out of which 34.4% had children under the age of 18 living with them, 58.0% were married couples living together, 8.2% had a female householder with no husband present, and 30.4% were non-families. 27.6% of all households were made up of individuals and 15.4% had someone living alone who was 65 years of age or older. The average household size was 2.59 and the average family size was 3.16.
In the village the population was spread out with 27.5% under the age of 18, 8.6% from 18 to 24, 28.8% from 25 to 44, 20.4% from 45 to 64, and 14.7% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 90.7 males. For every 100 females age 18 and over, there were 87.1 males.
The median income for a household in the village was $45,370, and the median income for a family was $151,591. Males had a median income of $140,156 versus $123,690 for females. The per capita income for the village was $119,110. About 2.2% of families and 5.4% of the population were below the poverty line, including 2.9% of those under age 18 and 10.5% of those age 65 or over.
The village of Somonauk is served by Somonauk Community Unit School District (CUSD) #432 which offers grades Pre-K–12. The school's athletic teams are known as the Bobcats. School colors are blue and gold. The school's song is titled "Bobcat Loyalty", and is a basic spin-off of the University of Illinois' fight song, named "Illinois Loyalty".
The community also shares the area known as "Lake Holiday" with neighboring Sandwich, with Somonauk School District covering most of the west side of the lake as well as a section of the southeast side. Sandwich School District covers most of the east side and a two small sections on the west side of the lake.
- "Geographic Names Information System". United States Geological Survey. Retrieved 2008-01-31.
- "Subcounty population estimates 2000-2006: Illinois" (CSV). United States Census Bureau, Population Division. 2007-06-28. Retrieved 2008-06-03.
- "G001 - Geographic Identifiers - 2010 Census Summary File 1". United States Census Bureau. Retrieved 2015-12-25.
- "Forest Preserve". DeKalb County Government. Retrieved February 15, 2013.
- "Lake Holiday". Official website. Lake Holiday Property Owners Association. 2013-02-14. Retrieved February 15, 2013.
- "Annual Estimates of the Resident Population for Incorporated Places: April 1, 2010 to July 1, 2014". Retrieved June 4, 2015.
- "Census of Population and Housing". Census.gov. Retrieved June 4, 2015.
- "American FactFinder". United States Census Bureau. Retrieved 2008-01-31.
- "Somonauk Community Unit School District 432". Somonauk CUSD 432. Retrieved 9 March 2014.