According to the United States Census Bureau, the city has a total area of, of which is land and is water. The Rum River flows through Isanti's western edge.
As of the census of 2010, there were 5,251 people, 1,871 households, and 1,336 families living in the city. The population density was. There were 2,038 housing units at an average density of. The racial makeup of the city was 94.7% White, 1.1% African American, 0.5% Native American, 0.7% Asian, 0.7% from other races, and 2.2% from two or more races. Hispanic or Latino of any race were 2.8% of the population. There were 1,871 households, of which 48.0% had children under the age of 18 living with them, 49.9% were married couplesliving together, 13.0% had a female householder with no husband present, 8.6% had a male householder with no wife present, and 28.6% were non-families. 20.6% of all households were made up of individuals, and 6.3% had someone living alone who was 65 years of age or older. The average household size was 2.81 and the average family size was 3.23. The median age in the city was 28.4 years. 32.8% of residents were under the age of 18; 8.4% were between the ages of 18 and 24; 36.9% were from 25 to 44; 15.4% were from 45 to 64; and 6.5% were 65 years of age or older. The gender makeup of the city was 50.4% male and 49.6% female.
2000 census
As of the census of 2000, there were 2,324 people, 816 households, and 576 families living in the city. The population density was 1,095.3 people per square mile. There were 834 housing units at an average density of 393.1 per square mile. The racial makeup of the city was 97.46% White, 0.56% African American, 0.65% Native American, 0.26% Asian, 0.04% Pacific Islander, 0.09% from other races, and 0.95% from two or more races. Hispanic or Latino of any race were 0.90% of the population. There were 816 households, out of which 48.7% had children under the age of 18 living with them, 49.6% were married couples living together, 16.2% had a female householder with no husband present, and 29.3% were non-families. 22.8% of all households were made up of individuals, and 8.5% had someone living alone who was 65 years of age or older. The average household size was 2.84 and the average family size was 3.31. In the city, the population was spread out, with 35.8% under the age of 18, 9.6% from 18 to 24, 35.3% from 25 to 44, 11.8% from 45 to 64, and 7.6% who were 65 years of age or older. The median age was 28 years. For every 100 females, there were 97.8 males. For every 100 females age 18 and over, there were 85.5 males. The median income for a household in the city was $43,587, and the median income for a family was $46,842. Males had a median income of $35,590 versus $25,081 for females. The per capita income for the city was $16,662. About 7.9% of families and 8.0% of the population were below the poverty line, including 9.9% of those under age 18 and 16.4% of those age 65 or over.
