| | Quincy, IL | Plainfield, IL | United States |
| Population | 39,687 | 44,296 | 329,725,481 |
| Female Population | 52.1% | 50.7% | 50.5% |
| Male Population | 47.9% | 49.4% | 49.5% |
| Median Age | 40 | 37.3 | 38.4 |
| space3 | | | |
| Population - 2010 | 40,770 | 39,990 | 309,327,143 |
| Population - 2000 | 40,366 | 13,038 | 285,036,114 |
| Population - 1990 | 39,681 | 4,557 | 251,960,433 |
| Pop. 2000 to Now | -1.9% | 241.6% | 15.6% |
| Pop. 1990 to Now | -0.2% | 877.4% | 30.8% |
| space4 | | | |
| Population Density | 2,525.9 | 1,808.1 | 93.4 |
| Land Area | 15.7 | 24.5 | 3,531,905.4 |
| Water Area | 0.0 | 1.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Quincy, IL | Plainfield, IL | United States |
| Age 0 to 4 | 6.3% | 5.7% | 5.9% |
| Age 5 to 9 | 6.4% | 8.2% | 6.1% |
| Age 10 to 14 | 6.1% | 10.5% | 6.6% |
| Age 15 to 17 | 2.8% | 5.8% | 3.9% |
| Age 18 to 20 | 3.0% | 3.8% | 4.0% |
| Age 21 to 24 | 5.4% | 4.3% | 5.2% |
| Age 25 to 34 | 14.0% | 8.7% | 13.8% |
| Age 35 to 44 | 11.8% | 15.6% | 12.9% |
| Age 45 to 54 | 10.9% | 17.9% | 12.6% |
| Age 55 to 59 | 6.5% | 6.0% | 6.7% |
| Age 60 to 64 | 6.5% | 4.3% | 6.3% |
| Age 65 to 74 | 10.8% | 5.6% | 9.6% |
| Age 75 to 84 | 5.8% | 2.4% | 4.5% |
| Age 85 and over | 3.7% | 1.3% | 1.9% |
| space1 | | | |
| Race | | | |
| | Quincy, IL | Plainfield, IL | United States |
| White | 87.4% | 68.6% | 59.4% |
| Black | 6.1% | 8.3% | 12.2% |
| Asian | 0.8% | 9.4% | 5.6% |
| Native American | 0.1% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.1% | 0.2% |
| Other | 0.0% | 0.3% | 0.4% |
| Two or More Races | 3.7% | 3.3% | 3.2% |
| Hispanic | 1.9% | 9.9% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Quincy, IL | Plainfield, IL | United States |
| Married Population | 48.1% | 62.9% | 49.8% |
| Currently Married | 46.4% | 62.1% | 48.0% |
| Married but Separated | 1.8% | 0.8% | 1.8% |
| Single Population | 51.9% | 37.1% | 50.2% |
| Never Married | 30.3% | 27.5% | 33.8% |
| Divorced | 13.9% | 6.5% | 10.8% |
| Widowed | 7.6% | 3.2% | 5.6% |
| space5 | | | |
| Household Size | 2.27 | 3.27 | 2.60 |
| Households | 17,016 | 13,510 | 124,010,992 |
| Family Households | 9,565 | 11,525 | 80,755,759 |
| Married couple, w/children | 25.8% | 50.1% | 30.5% |
| Married couple, no children | 46.1% | 34.6% | 42.9% |
| Non Family Households | 7,451 | 1,985 | 43,255,233 |
| Single householder, w/children | 18.5% | 9.3% | 15.8% |
| Single householder, no children | 9.6% | 6.0% | 10.8% |