| | Romeoville, IL | Quincy, IL | United States |
| Population | 40,099 | 39,687 | 329,725,481 |
| Female Population | 50.8% | 52.1% | 50.5% |
| Male Population | 49.2% | 47.9% | 49.5% |
| Median Age | 35.5 | 40 | 38.4 |
| space3 | | | |
| Population - 2010 | 39,697 | 40,770 | 309,327,143 |
| Population - 2000 | 21,153 | 40,366 | 285,036,114 |
| Population - 1990 | 14,074 | 39,681 | 251,960,433 |
| Pop. 2000 to Now | 88.0% | -1.9% | 15.6% |
| Pop. 1990 to Now | 182.6% | -0.2% | 30.8% |
| space4 | | | |
| Population Density | 2,109.0 | 2,525.9 | 93.4 |
| Land Area | 19.0 | 15.7 | 3,531,905.4 |
| Water Area | 1.0 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Romeoville, IL | Quincy, IL | United States |
| Age 0 to 4 | 5.8% | 6.3% | 5.9% |
| Age 5 to 9 | 5.7% | 6.4% | 6.1% |
| Age 10 to 14 | 6.4% | 6.1% | 6.6% |
| Age 15 to 17 | 5.6% | 2.8% | 3.9% |
| Age 18 to 20 | 6.3% | 3.0% | 4.0% |
| Age 21 to 24 | 6.7% | 5.4% | 5.2% |
| Age 25 to 34 | 12.8% | 14.0% | 13.8% |
| Age 35 to 44 | 13.6% | 11.8% | 12.9% |
| Age 45 to 54 | 14.9% | 10.9% | 12.6% |
| Age 55 to 59 | 6.7% | 6.5% | 6.7% |
| Age 60 to 64 | 4.6% | 6.5% | 6.3% |
| Age 65 to 74 | 6.4% | 10.8% | 9.6% |
| Age 75 to 84 | 3.2% | 5.8% | 4.5% |
| Age 85 and over | 1.3% | 3.7% | 1.9% |
| space1 | | | |
| Race | | | |
| | Romeoville, IL | Quincy, IL | United States |
| White | 45.7% | 87.4% | 59.4% |
| Black | 11.1% | 6.1% | 12.2% |
| Asian | 5.8% | 0.8% | 5.6% |
| Native American | 0.0% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.2% | 0.0% | 0.4% |
| Two or More Races | 3.3% | 3.7% | 3.2% |
| Hispanic | 33.8% | 1.9% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Romeoville, IL | Quincy, IL | United States |
| Married Population | 50.1% | 48.1% | 49.8% |
| Currently Married | 48.8% | 46.4% | 48.0% |
| Married but Separated | 1.3% | 1.8% | 1.8% |
| Single Population | 49.9% | 51.9% | 50.2% |
| Never Married | 36.7% | 30.3% | 33.8% |
| Divorced | 8.3% | 13.9% | 10.8% |
| Widowed | 4.8% | 7.6% | 5.6% |
| space5 | | | |
| Household Size | 3.24 | 2.27 | 2.60 |
| Households | 12,047 | 17,016 | 124,010,992 |
| Family Households | 9,079 | 9,565 | 80,755,759 |
| Married couple, w/children | 40.1% | 25.8% | 30.5% |
| Married couple, no children | 38.9% | 46.1% | 42.9% |
| Non Family Households | 2,968 | 7,451 | 43,255,233 |
| Single householder, w/children | 11.4% | 18.5% | 15.8% |
| Single householder, no children | 9.5% | 9.6% | 10.8% |