| | Springfield, IL | Decatur, IL | United States |
| Population | 114,672 | 71,309 | 329,725,481 |
| Female Population | 52.1% | 52.7% | 50.5% |
| Male Population | 47.9% | 47.3% | 49.5% |
| Median Age | 39.7 | 39.9 | 38.4 |
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| Population - 2010 | 117,139 | 76,095 | 309,327,143 |
| Population - 2000 | 111,454 | 81,860 | 285,036,114 |
| Population - 1990 | 105,227 | 83,885 | 251,960,433 |
| Pop. 2000 to Now | 2.0% | -14.5% | 15.6% |
| Pop. 1990 to Now | 8.0% | -16.5% | 30.8% |
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| Population Density | 1,880.5 | 1,685.0 | 93.4 |
| Land Area | 61.0 | 42.3 | 3,531,905.4 |
| Water Area | 6.3 | 4.7 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Springfield, IL | Decatur, IL | United States |
| Age 0 to 4 | 6.1% | 6.0% | 5.9% |
| Age 5 to 9 | 6.3% | 5.5% | 6.1% |
| Age 10 to 14 | 6.0% | 5.7% | 6.6% |
| Age 15 to 17 | 3.8% | 3.7% | 3.9% |
| Age 18 to 20 | 3.9% | 4.7% | 4.0% |
| Age 21 to 24 | 4.9% | 5.2% | 5.2% |
| Age 25 to 34 | 13.3% | 13.2% | 13.8% |
| Age 35 to 44 | 11.9% | 11.3% | 12.9% |
| Age 45 to 54 | 12.1% | 10.8% | 12.6% |
| Age 55 to 59 | 7.5% | 7.0% | 6.7% |
| Age 60 to 64 | 6.5% | 6.8% | 6.3% |
| Age 65 to 74 | 10.5% | 11.1% | 9.6% |
| Age 75 to 84 | 5.2% | 5.7% | 4.5% |
| Age 85 and over | 2.1% | 3.3% | 1.9% |
| space1 | | | |
| Race | | | |
| | Springfield, IL | Decatur, IL | United States |
| White | 70.4% | 67.5% | 59.4% |
| Black | 19.9% | 23.0% | 12.2% |
| Asian | 3.2% | 1.4% | 5.6% |
| Native American | 0.1% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.1% | 0.2% |
| Other | 0.2% | 0.1% | 0.4% |
| Two or More Races | 3.3% | 4.9% | 3.2% |
| Hispanic | 2.9% | 2.8% | 18.4% |
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| Marriage & Family | | | |
| | Springfield, IL | Decatur, IL | United States |
| Married Population | 41.0% | 42.5% | 49.8% |
| Currently Married | 39.5% | 41.1% | 48.0% |
| Married but Separated | 1.6% | 1.4% | 1.8% |
| Single Population | 59.0% | 57.5% | 50.2% |
| Never Married | 37.6% | 35.7% | 33.8% |
| Divorced | 15.1% | 14.0% | 10.8% |
| Widowed | 6.2% | 7.8% | 5.6% |
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| Household Size | 2.18 | 2.21 | 2.60 |
| Households | 50,961 | 30,718 | 124,010,992 |
| Family Households | 27,450 | 16,335 | 80,755,759 |
| Married couple, w/children | 21.6% | 20.5% | 30.5% |
| Married couple, no children | 41.8% | 48.1% | 42.9% |
| Non Family Households | 23,511 | 14,383 | 43,255,233 |
| Single householder, w/children | 24.6% | 20.0% | 15.8% |
| Single householder, no children | 11.9% | 11.4% | 10.8% |