| | Decatur, IL | Belleville, IL | United States |
| Population | 71,309 | 42,463 | 329,725,481 |
| Female Population | 52.7% | 52.3% | 50.5% |
| Male Population | 47.3% | 47.8% | 49.5% |
| Median Age | 39.9 | 37.6 | 38.4 |
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| Population - 2010 | 76,095 | 44,230 | 309,327,143 |
| Population - 2000 | 81,860 | 41,410 | 285,036,114 |
| Population - 1990 | 83,885 | 42,785 | 251,960,433 |
| Pop. 2000 to Now | -14.5% | -2.1% | 15.6% |
| Pop. 1990 to Now | -16.5% | -5.3% | 30.8% |
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| Population Density | 1,685.0 | 1,830.9 | 93.4 |
| Land Area | 42.3 | 23.2 | 3,531,905.4 |
| Water Area | 4.7 | 0.3 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Decatur, IL | Belleville, IL | United States |
| Age 0 to 4 | 6.0% | 6.5% | 5.9% |
| Age 5 to 9 | 5.5% | 6.0% | 6.1% |
| Age 10 to 14 | 5.7% | 7.4% | 6.6% |
| Age 15 to 17 | 3.7% | 4.1% | 3.9% |
| Age 18 to 20 | 4.7% | 2.9% | 4.0% |
| Age 21 to 24 | 5.2% | 6.0% | 5.2% |
| Age 25 to 34 | 13.2% | 13.5% | 13.8% |
| Age 35 to 44 | 11.3% | 13.5% | 12.9% |
| Age 45 to 54 | 10.8% | 13.0% | 12.6% |
| Age 55 to 59 | 7.0% | 6.0% | 6.7% |
| Age 60 to 64 | 6.8% | 7.0% | 6.3% |
| Age 65 to 74 | 11.1% | 9.0% | 9.6% |
| Age 75 to 84 | 5.7% | 3.0% | 4.5% |
| Age 85 and over | 3.3% | 2.1% | 1.9% |
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| Race | | | |
| | Decatur, IL | Belleville, IL | United States |
| White | 67.5% | 58.4% | 59.4% |
| Black | 23.0% | 31.3% | 12.2% |
| Asian | 1.4% | 0.5% | 5.6% |
| Native American | 0.2% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.1% | 0.1% | 0.2% |
| Other | 0.1% | 0.3% | 0.4% |
| Two or More Races | 4.9% | 6.0% | 3.2% |
| Hispanic | 2.8% | 3.5% | 18.4% |
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| Marriage & Family | | | |
| | Decatur, IL | Belleville, IL | United States |
| Married Population | 42.5% | 40.5% | 49.8% |
| Currently Married | 41.1% | 38.0% | 48.0% |
| Married but Separated | 1.4% | 2.4% | 1.8% |
| Single Population | 57.5% | 59.5% | 50.2% |
| Never Married | 35.7% | 40.7% | 33.8% |
| Divorced | 14.0% | 13.2% | 10.8% |
| Widowed | 7.8% | 5.7% | 5.6% |
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| Household Size | 2.21 | 2.34 | 2.60 |
| Households | 30,718 | 17,589 | 124,010,992 |
| Family Households | 16,335 | 9,589 | 80,755,759 |
| Married couple, w/children | 20.5% | 26.2% | 30.5% |
| Married couple, no children | 48.1% | 37.9% | 42.9% |
| Non Family Households | 14,383 | 8,000 | 43,255,233 |
| Single householder, w/children | 20.0% | 25.9% | 15.8% |
| Single householder, no children | 11.4% | 10.1% | 10.8% |