| | Algonquin, IL | Decatur, IL | United States |
| Population | 29,869 | 71,309 | 329,725,481 |
| Female Population | 49.5% | 52.7% | 50.5% |
| Male Population | 50.5% | 47.3% | 49.5% |
| Median Age | 42 | 39.9 | 38.4 |
| space3 | | | |
| Population - 2010 | 30,079 | 76,095 | 309,327,143 |
| Population - 2000 | 23,276 | 81,860 | 285,036,114 |
| Population - 1990 | 11,663 | 83,885 | 251,960,433 |
| Pop. 2000 to Now | 32.0% | -14.5% | 15.6% |
| Pop. 1990 to Now | 163.5% | -16.5% | 30.8% |
| space4 | | | |
| Population Density | 2,442.1 | 1,685.0 | 93.4 |
| Land Area | 12.2 | 42.3 | 3,531,905.4 |
| Water Area | 0.2 | 4.7 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Algonquin, IL | Decatur, IL | United States |
| Age 0 to 4 | 4.7% | 6.0% | 5.9% |
| Age 5 to 9 | 6.5% | 5.5% | 6.1% |
| Age 10 to 14 | 5.9% | 5.7% | 6.6% |
| Age 15 to 17 | 4.8% | 3.7% | 3.9% |
| Age 18 to 20 | 3.9% | 4.7% | 4.0% |
| Age 21 to 24 | 4.6% | 5.2% | 5.2% |
| Age 25 to 34 | 12.0% | 13.2% | 13.8% |
| Age 35 to 44 | 11.8% | 11.3% | 12.9% |
| Age 45 to 54 | 16.3% | 10.8% | 12.6% |
| Age 55 to 59 | 9.8% | 7.0% | 6.7% |
| Age 60 to 64 | 6.8% | 6.8% | 6.3% |
| Age 65 to 74 | 7.5% | 11.1% | 9.6% |
| Age 75 to 84 | 3.4% | 5.7% | 4.5% |
| Age 85 and over | 2.2% | 3.3% | 1.9% |
| space1 | | | |
| Race | | | |
| | Algonquin, IL | Decatur, IL | United States |
| White | 77.3% | 67.5% | 59.4% |
| Black | 2.5% | 23.0% | 12.2% |
| Asian | 6.8% | 1.4% | 5.6% |
| Native American | 0.0% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.1% | 0.2% |
| Other | 0.4% | 0.1% | 0.4% |
| Two or More Races | 2.8% | 4.9% | 3.2% |
| Hispanic | 10.2% | 2.8% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Algonquin, IL | Decatur, IL | United States |
| Married Population | 59.0% | 42.5% | 49.8% |
| Currently Married | 58.1% | 41.1% | 48.0% |
| Married but Separated | 0.9% | 1.4% | 1.8% |
| Single Population | 41.0% | 57.5% | 50.2% |
| Never Married | 27.7% | 35.7% | 33.8% |
| Divorced | 8.1% | 14.0% | 10.8% |
| Widowed | 5.3% | 7.8% | 5.6% |
| space5 | | | |
| Household Size | 2.71 | 2.21 | 2.60 |
| Households | 11,018 | 30,718 | 124,010,992 |
| Family Households | 8,472 | 16,335 | 80,755,759 |
| Married couple, w/children | 32.4% | 20.5% | 30.5% |
| Married couple, no children | 49.4% | 48.1% | 42.9% |
| Non Family Households | 2,546 | 14,383 | 43,255,233 |
| Single householder, w/children | 10.0% | 20.0% | 15.8% |
| Single householder, no children | 8.3% | 11.4% | 10.8% |