| | Marion, IL | Elgin, IL | United States |
| Population | 16,960 | 114,156 | 329,725,481 |
| Female Population | 50.7% | 49.3% | 50.5% |
| Male Population | 49.3% | 50.7% | 49.5% |
| Median Age | 40.5 | 35 | 38.4 |
| space3 | | | |
| Population - 2010 | 17,231 | 108,386 | 309,327,143 |
| Population - 2000 | 16,035 | 94,487 | 285,036,114 |
| Population - 1990 | 14,545 | 77,010 | 251,960,433 |
| Pop. 2000 to Now | 8.6% | 16.6% | 15.6% |
| Pop. 1990 to Now | 19.8% | 43.1% | 30.8% |
| space4 | | | |
| Population Density | 1,029.4 | 3,044.1 | 93.4 |
| Land Area | 16.5 | 37.5 | 3,531,905.4 |
| Water Area | 0.2 | 0.6 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Marion, IL | Elgin, IL | United States |
| Age 0 to 4 | 5.0% | 7.6% | 5.9% |
| Age 5 to 9 | 6.4% | 6.8% | 6.1% |
| Age 10 to 14 | 6.8% | 7.4% | 6.6% |
| Age 15 to 17 | 3.3% | 4.9% | 3.9% |
| Age 18 to 20 | 2.7% | 4.1% | 4.0% |
| Age 21 to 24 | 3.5% | 5.3% | 5.2% |
| Age 25 to 34 | 13.9% | 13.8% | 13.8% |
| Age 35 to 44 | 15.1% | 12.9% | 12.9% |
| Age 45 to 54 | 10.7% | 12.9% | 12.6% |
| Age 55 to 59 | 4.1% | 6.7% | 6.7% |
| Age 60 to 64 | 5.0% | 6.0% | 6.3% |
| Age 65 to 74 | 13.9% | 7.3% | 9.6% |
| Age 75 to 84 | 7.0% | 3.1% | 4.5% |
| Age 85 and over | 2.5% | 1.1% | 1.9% |
| space1 | | | |
| Race | | | |
| | Marion, IL | Elgin, IL | United States |
| White | 85.8% | 39.0% | 59.4% |
| Black | 8.0% | 5.0% | 12.2% |
| Asian | 1.7% | 5.2% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.1% | 0.6% | 0.4% |
| Two or More Races | 2.7% | 2.7% | 3.2% |
| Hispanic | 1.7% | 47.4% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Marion, IL | Elgin, IL | United States |
| Married Population | 42.7% | 52.0% | 49.8% |
| Currently Married | 41.5% | 50.0% | 48.0% |
| Married but Separated | 1.2% | 2.0% | 1.8% |
| Single Population | 57.3% | 48.0% | 50.2% |
| Never Married | 32.8% | 35.0% | 33.8% |
| Divorced | 16.1% | 9.3% | 10.8% |
| Widowed | 8.3% | 3.7% | 5.6% |
| space5 | | | |
| Household Size | 2.07 | 2.94 | 2.60 |
| Households | 7,994 | 38,168 | 124,010,992 |
| Family Households | 4,083 | 27,370 | 80,755,759 |
| Married couple, w/children | 21.7% | 35.8% | 30.5% |
| Married couple, no children | 46.4% | 37.0% | 42.9% |
| Non Family Households | 3,911 | 10,798 | 43,255,233 |
| Single householder, w/children | 23.0% | 16.0% | 15.8% |
| Single householder, no children | 8.8% | 11.2% | 10.8% |