| | Marengo, IL | McHenry, IL | United States |
| Population | 7,466 | 27,237 | 329,725,481 |
| Female Population | 50.7% | 51.1% | 50.5% |
| Male Population | 49.3% | 48.9% | 49.5% |
| Median Age | 30.5 | 42.2 | 38.4 |
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| Population - 2010 | 7,672 | 27,045 | 309,327,143 |
| Population - 2000 | 6,355 | 21,501 | 285,036,114 |
| Population - 1990 | 4,768 | 16,177 | 251,960,433 |
| Pop. 2000 to Now | 16.1% | 26.0% | 15.6% |
| Pop. 1990 to Now | 54.7% | 67.5% | 30.8% |
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| Population Density | 944.4 | 1,864.7 | 93.4 |
| Land Area | 7.9 | 14.6 | 3,531,905.4 |
| Water Area | 0.0 | 0.4 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Marengo, IL | McHenry, IL | United States |
| Age 0 to 4 | 7.8% | 5.2% | 5.9% |
| Age 5 to 9 | 7.1% | 6.2% | 6.1% |
| Age 10 to 14 | 6.7% | 5.4% | 6.6% |
| Age 15 to 17 | 4.0% | 3.5% | 3.9% |
| Age 18 to 20 | 2.5% | 3.1% | 4.0% |
| Age 21 to 24 | 8.7% | 4.7% | 5.2% |
| Age 25 to 34 | 17.4% | 12.7% | 13.8% |
| Age 35 to 44 | 7.7% | 14.2% | 12.9% |
| Age 45 to 54 | 9.8% | 14.1% | 12.6% |
| Age 55 to 59 | 5.9% | 7.6% | 6.7% |
| Age 60 to 64 | 8.0% | 6.3% | 6.3% |
| Age 65 to 74 | 6.5% | 10.0% | 9.6% |
| Age 75 to 84 | 5.0% | 3.8% | 4.5% |
| Age 85 and over | 2.8% | 3.3% | 1.9% |
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| Race | | | |
| | Marengo, IL | McHenry, IL | United States |
| White | 81.1% | 81.0% | 59.4% |
| Black | 2.6% | 0.6% | 12.2% |
| Asian | 0.0% | 0.9% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.0% | 0.4% |
| Two or More Races | 0.7% | 1.5% | 3.2% |
| Hispanic | 15.6% | 16.1% | 18.4% |
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| Marriage & Family | | | |
| | Marengo, IL | McHenry, IL | United States |
| Married Population | 46.1% | 55.4% | 49.8% |
| Currently Married | 44.8% | 54.8% | 48.0% |
| Married but Separated | 1.3% | 0.7% | 1.8% |
| Single Population | 53.9% | 44.6% | 50.2% |
| Never Married | 38.3% | 25.7% | 33.8% |
| Divorced | 6.2% | 12.5% | 10.8% |
| Widowed | 9.3% | 6.4% | 5.6% |
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| Household Size | 2.56 | 2.45 | 2.60 |
| Households | 2,895 | 11,018 | 124,010,992 |
| Family Households | 1,961 | 7,350 | 80,755,759 |
| Married couple, w/children | 27.2% | 29.3% | 30.5% |
| Married couple, no children | 37.4% | 51.1% | 42.9% |
| Non Family Households | 934 | 3,668 | 43,255,233 |
| Single householder, w/children | 29.3% | 12.9% | 15.8% |
| Single householder, no children | 6.1% | 6.7% | 10.8% |