Colchester is a city in McDonough County, Illinois, United States. The population was 1,493 at the 2000 census. The city is named after the town of Colchester, England.
Colchester is located at 40°25′31″N 90°47′32″W / 40.42528°N 90.79222°W / 40.42528; -90.79222 (40.425174, -90.792339).
According to the 2010 census, Colchester has a total area of 1 square mile (2.59 km), all land.
Colchester is the site of Argyle Lake State Park.
At the 2000 census, there were 1,493 people, 634 households and 419 families residing in the city. The population density was 1,500.7 per square mile (582.3/km²). There were 694 housing units at an average density of 697.6 per square mile (270.7/km²). The racial makeup of the city was 99.40% White, 0.07% Native American, 0.07% Asian, and 0.47% from two or more races. Hispanic or Latino of any race were 0.60% of the population.
There were 634 households of which 29.8% had children under the age of 18 living with them, 50.8% were married couples living together, 12.1% had a female householder with no husband present, and 33.8% were non-families. 30.6% of all households were made up of individuals and 13.2% had someone living alone who was 65 years of age or older. The average household size was 2.32 and the average family size was 2.89.
22.9% of the population were under the age of 18, 9.8% from 18 to 24, 27.6% from 25 to 44, 22.6% from 45 to 64, and 17.0% who were 65 years of age or older. The median age was 38 years. For every 100 females, there were 85.9 males. For every 100 females age 18 and over, there were 78.4 males.
The median household income was $31,283 and the median family income was $37,763. Males had a median income of $27,857 compared with $19,211 for females. The per capita income for the city was $15,354. About 5.0% of families and 7.3% of the population were below the poverty line, including 8.8% of those under age 18 and 5.9% of those age 65 or over.