| | South Beloit, IL | Lanark, IL | United States |
| Population | 7,943 | 1,571 | 329,725,481 |
| Female Population | 49.4% | 51.9% | 50.5% |
| Male Population | 50.6% | 48.1% | 49.5% |
| Median Age | 32.1 | 43.4 | 38.4 |
| space3 | | | |
| Population - 2010 | 7,853 | 1,459 | 309,327,143 |
| Population - 2000 | 5,397 | 1,584 | 285,036,114 |
| Population - 1990 | 4,072 | 1,382 | 251,960,433 |
| Pop. 2000 to Now | 39.7% | -16.6% | 15.6% |
| Pop. 1990 to Now | 85.2% | -4.4% | 30.8% |
| space4 | | | |
| Population Density | 1,313.1 | 1,396.4 | 93.4 |
| Land Area | 6.0 | 1.1 | 3,531,905.4 |
| Water Area | 0.2 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | South Beloit, IL | Lanark, IL | United States |
| Age 0 to 4 | 8.8% | 7.0% | 5.9% |
| Age 5 to 9 | 8.9% | 10.0% | 6.1% |
| Age 10 to 14 | 6.5% | 4.5% | 6.6% |
| Age 15 to 17 | 2.9% | 0.9% | 3.9% |
| Age 18 to 20 | 2.7% | 4.1% | 4.0% |
| Age 21 to 24 | 3.7% | 4.6% | 5.2% |
| Age 25 to 34 | 21.7% | 8.8% | 13.8% |
| Age 35 to 44 | 11.3% | 14.0% | 12.9% |
| Age 45 to 54 | 11.7% | 11.0% | 12.6% |
| Age 55 to 59 | 5.9% | 8.9% | 6.7% |
| Age 60 to 64 | 4.6% | 5.4% | 6.3% |
| Age 65 to 74 | 6.9% | 11.8% | 9.6% |
| Age 75 to 84 | 1.8% | 7.3% | 4.5% |
| Age 85 and over | 2.5% | 1.6% | 1.9% |
| space1 | | | |
| Race | | | |
| | South Beloit, IL | Lanark, IL | United States |
| White | 86.0% | 93.3% | 59.4% |
| Black | 3.0% | 1.5% | 12.2% |
| Asian | 0.1% | 0.0% | 5.6% |
| Native American | 0.0% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.0% | 0.4% |
| Two or More Races | 4.9% | 1.8% | 3.2% |
| Hispanic | 6.0% | 3.2% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | South Beloit, IL | Lanark, IL | United States |
| Married Population | 56.7% | 54.5% | 49.8% |
| Currently Married | 55.1% | 54.3% | 48.0% |
| Married but Separated | 1.6% | 0.2% | 1.8% |
| Single Population | 43.3% | 45.5% | 50.2% |
| Never Married | 27.0% | 30.0% | 33.8% |
| Divorced | 10.9% | 9.7% | 10.8% |
| Widowed | 5.4% | 5.8% | 5.6% |
| space5 | | | |
| Household Size | 2.46 | 2.52 | 2.60 |
| Households | 3,197 | 617 | 124,010,992 |
| Family Households | 1,823 | 421 | 80,755,759 |
| Married couple, w/children | 47.8% | 28.0% | 30.5% |
| Married couple, no children | 38.7% | 49.2% | 42.9% |
| Non Family Households | 1,374 | 196 | 43,255,233 |
| Single householder, w/children | 7.7% | 16.2% | 15.8% |
| Single householder, no children | 5.7% | 6.7% | 10.8% |