| | Frankfort, IL | Flossmoor, IL | United States |
| Population | 20,040 | 9,868 | 329,725,481 |
| Female Population | 50.6% | 52.9% | 50.5% |
| Male Population | 49.4% | 47.2% | 49.5% |
| Median Age | 41.9 | 43 | 38.4 |
| space3 | | | |
| Population - 2010 | 17,827 | 9,421 | 309,327,143 |
| Population - 2000 | 10,391 | 9,301 | 285,036,114 |
| Population - 1990 | 7,180 | 8,651 | 251,960,433 |
| Pop. 2000 to Now | 86.8% | -3.0% | 15.6% |
| Pop. 1990 to Now | 170.4% | 4.2% | 30.8% |
| space4 | | | |
| Population Density | 1,246.5 | 2,695.4 | 93.4 |
| Land Area | 16.1 | 3.7 | 3,531,905.4 |
| Water Area | 0.0 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Frankfort, IL | Flossmoor, IL | United States |
| Age 0 to 4 | 4.6% | 7.9% | 5.9% |
| Age 5 to 9 | 9.1% | 6.7% | 6.1% |
| Age 10 to 14 | 8.6% | 6.2% | 6.6% |
| Age 15 to 17 | 6.0% | 5.2% | 3.9% |
| Age 18 to 20 | 2.5% | 4.8% | 4.0% |
| Age 21 to 24 | 3.5% | 2.0% | 5.2% |
| Age 25 to 34 | 7.4% | 5.8% | 13.8% |
| Age 35 to 44 | 11.3% | 12.7% | 12.9% |
| Age 45 to 54 | 17.4% | 16.0% | 12.6% |
| Age 55 to 59 | 6.8% | 7.1% | 6.7% |
| Age 60 to 64 | 7.1% | 8.2% | 6.3% |
| Age 65 to 74 | 11.2% | 12.2% | 9.6% |
| Age 75 to 84 | 3.9% | 3.2% | 4.5% |
| Age 85 and over | 0.6% | 2.1% | 1.9% |
| space1 | | | |
| Race | | | |
| | Frankfort, IL | Flossmoor, IL | United States |
| White | 80.8% | 28.6% | 59.4% |
| Black | 8.7% | 64.1% | 12.2% |
| Asian | 3.1% | 1.9% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.1% | 0.4% |
| Two or More Races | 1.9% | 3.6% | 3.2% |
| Hispanic | 5.4% | 1.6% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Frankfort, IL | Flossmoor, IL | United States |
| Married Population | 67.0% | 59.7% | 49.8% |
| Currently Married | 66.5% | 57.8% | 48.0% |
| Married but Separated | 0.4% | 1.8% | 1.8% |
| Single Population | 33.0% | 40.3% | 50.2% |
| Never Married | 24.7% | 26.9% | 33.8% |
| Divorced | 4.7% | 8.2% | 10.8% |
| Widowed | 3.6% | 5.2% | 5.6% |
| space5 | | | |
| Household Size | 3.11 | 2.79 | 2.60 |
| Households | 6,405 | 3,519 | 124,010,992 |
| Family Households | 5,526 | 2,621 | 80,755,759 |
| Married couple, w/children | 44.8% | 37.6% | 30.5% |
| Married couple, no children | 46.1% | 45.9% | 42.9% |
| Non Family Households | 879 | 898 | 43,255,233 |
| Single householder, w/children | 5.4% | 13.7% | 15.8% |
| Single householder, no children | 3.8% | 2.7% | 10.8% |