| | Mendota, IL | Belvidere, IL | United States |
| Population | 6,996 | 25,179 | 329,725,481 |
| Female Population | 51.2% | 52.3% | 50.5% |
| Male Population | 48.8% | 47.7% | 49.5% |
| Median Age | 38.4 | 36.9 | 38.4 |
| space3 | | | |
| Population - 2010 | 7,372 | 25,598 | 309,327,143 |
| Population - 2000 | 7,272 | 20,820 | 285,036,114 |
| Population - 1990 | 7,018 | 15,958 | 251,960,433 |
| Pop. 2000 to Now | -5.1% | 18.8% | 15.6% |
| Pop. 1990 to Now | -1.6% | 55.0% | 30.8% |
| space4 | | | |
| Population Density | 1,383.7 | 2,085.9 | 93.4 |
| Land Area | 5.1 | 12.1 | 3,531,905.4 |
| Water Area | 0.1 | 0.2 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Mendota, IL | Belvidere, IL | United States |
| Age 0 to 4 | 5.6% | 5.9% | 5.9% |
| Age 5 to 9 | 6.9% | 7.1% | 6.1% |
| Age 10 to 14 | 6.6% | 7.2% | 6.6% |
| Age 15 to 17 | 5.6% | 3.6% | 3.9% |
| Age 18 to 20 | 3.3% | 4.5% | 4.0% |
| Age 21 to 24 | 7.6% | 4.5% | 5.2% |
| Age 25 to 34 | 10.6% | 13.4% | 13.8% |
| Age 35 to 44 | 12.5% | 11.9% | 12.9% |
| Age 45 to 54 | 11.9% | 13.5% | 12.6% |
| Age 55 to 59 | 6.9% | 7.5% | 6.7% |
| Age 60 to 64 | 3.4% | 4.8% | 6.3% |
| Age 65 to 74 | 11.1% | 10.1% | 9.6% |
| Age 75 to 84 | 5.0% | 4.4% | 4.5% |
| Age 85 and over | 3.2% | 1.4% | 1.9% |
| space1 | | | |
| Race | | | |
| | Mendota, IL | Belvidere, IL | United States |
| White | 71.0% | 60.1% | 59.4% |
| Black | 0.4% | 1.3% | 12.2% |
| Asian | 1.5% | 0.9% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.2% | 0.4% |
| Two or More Races | 0.6% | 1.1% | 3.2% |
| Hispanic | 26.5% | 36.4% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Mendota, IL | Belvidere, IL | United States |
| Married Population | 49.5% | 42.0% | 49.8% |
| Currently Married | 47.0% | 39.9% | 48.0% |
| Married but Separated | 2.6% | 2.1% | 1.8% |
| Single Population | 50.5% | 58.0% | 50.2% |
| Never Married | 33.4% | 36.6% | 33.8% |
| Divorced | 10.1% | 14.5% | 10.8% |
| Widowed | 7.0% | 6.9% | 5.6% |
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| Household Size | 2.54 | 2.71 | 2.60 |
| Households | 2,690 | 9,193 | 124,010,992 |
| Family Households | 1,756 | 5,829 | 80,755,759 |
| Married couple, w/children | 30.9% | 26.5% | 30.5% |
| Married couple, no children | 41.2% | 37.6% | 42.9% |
| Non Family Households | 934 | 3,364 | 43,255,233 |
| Single householder, w/children | 20.3% | 23.0% | 15.8% |
| Single householder, no children | 7.6% | 12.9% | 10.8% |