As of the census of 2000, there were 40,631 people, 14,910 households, and 11,167 families residing in the county. The population density was 127 people per square mile (49/km²). There were 15,758 housing units at an average density of 49 per square mile (19/km²). The racial makeup of the county was 96.68% White, 0.31% Black or African American, 0.34% Native American, 1.55% Asian, 0.01% Pacific Islander, 0.38% from other races, and 0.74% from two or more races. 1.07% of the population were Hispanic or Latino of any race. 63.4% were of German ancestry according to Census 2000. 96.0% spoke English, 1.7% Spanish and 1.2% German as their first language.
There were 14,910 households out of which 38.50% had children under the age of 18 living with them, 65.00% were married couples living together, 6.50% had a female householder with no husband present, and 25.10% were non-families. 20.40% of all households were made up of individuals and 7.50% had someone living alone who was 65 years of age or older. The average household size was 2.70 and the average family size was 3.15.
In the county, the population was spread out with 28.60% under the age of 18, 7.20% from 18 to 24, 32.00% from 25 to 44, 21.40% from 45 to 64, and 10.80% who were 65 years of age or older. The median age was 35 years. For every 100 females there were 100.00 males. For every 100 females age 18 and over, there were 99.00 males.
No interstate highways run through Calumet County. There are several U.S. routes in the county. U.S. Route 10 runs east-west across the north edge of the county. U.S. Route 151 runs north-south near the west edge of the county, and turns east-west at the middle of the county. North-south state highways include 55 along the west edge, and 32/57 through the center. East-west state highways include 114 at the northwest corner of the county, and the now defunct 149 along the southeast corner.
The only airport located in Calumet County is the small New Holstein Municipal Airport. It is located just west of New Holstein.
Nearly the entire west boundary of the county is located in Lake Winnebago. The first non-natives to enter the county most likely came in the county from the lake through the Fox River. The Manitowoc River and Sheboygan River both run through the county and flow into Lake Michigan through neighboring counties.