| | Union, SC | Rock Hill, SC | United States |
| Population | 8,119 | 72,883 | 329,725,481 |
| Female Population | 54.7% | 54.1% | 50.5% |
| Male Population | 45.3% | 45.9% | 49.5% |
| Median Age | 46.1 | 34.1 | 38.4 |
| space3 | | | |
| Population - 2010 | 8,352 | 66,872 | 309,327,143 |
| Population - 2000 | 8,793 | 49,765 | 285,036,114 |
| Population - 1990 | 9,836 | 41,643 | 251,960,433 |
| Pop. 2000 to Now | -14.3% | 53.4% | 15.6% |
| Pop. 1990 to Now | -23.4% | 83.3% | 30.8% |
| space4 | | | |
| Population Density | 1,017.2 | 1,916.6 | 93.4 |
| Land Area | 8.0 | 38.0 | 3,531,905.4 |
| Water Area | 0.0 | 0.2 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Union, SC | Rock Hill, SC | United States |
| Age 0 to 4 | 5.8% | 6.4% | 5.9% |
| Age 5 to 9 | 5.3% | 6.4% | 6.1% |
| Age 10 to 14 | 4.2% | 6.3% | 6.6% |
| Age 15 to 17 | 2.5% | 4.0% | 3.9% |
| Age 18 to 20 | 4.1% | 5.9% | 4.0% |
| Age 21 to 24 | 4.1% | 6.8% | 5.2% |
| Age 25 to 34 | 11.0% | 15.7% | 13.8% |
| Age 35 to 44 | 10.0% | 11.8% | 12.9% |
| Age 45 to 54 | 15.4% | 11.8% | 12.6% |
| Age 55 to 59 | 8.1% | 5.5% | 6.7% |
| Age 60 to 64 | 8.0% | 5.6% | 6.3% |
| Age 65 to 74 | 13.2% | 8.7% | 9.6% |
| Age 75 to 84 | 5.1% | 3.6% | 4.5% |
| Age 85 and over | 3.3% | 1.6% | 1.9% |
| space1 | | | |
| Race | | | |
| | Union, SC | Rock Hill, SC | United States |
| White | 49.1% | 51.1% | 59.4% |
| Black | 45.7% | 38.5% | 12.2% |
| Asian | 0.0% | 1.9% | 5.6% |
| Native American | 0.1% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.4% | 0.4% |
| Two or More Races | 1.9% | 2.2% | 3.2% |
| Hispanic | 3.2% | 5.7% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Union, SC | Rock Hill, SC | United States |
| Married Population | 40.3% | 42.7% | 49.8% |
| Currently Married | 37.2% | 39.8% | 48.0% |
| Married but Separated | 3.2% | 2.9% | 1.8% |
| Single Population | 59.7% | 57.3% | 50.2% |
| Never Married | 36.2% | 40.9% | 33.8% |
| Divorced | 13.5% | 10.8% | 10.8% |
| Widowed | 10.0% | 5.7% | 5.6% |
| space5 | | | |
| Household Size | 2.15 | 2.37 | 2.60 |
| Households | 3,716 | 29,588 | 124,010,992 |
| Family Households | 2,019 | 17,390 | 80,755,759 |
| Married couple, w/children | 18.3% | 24.7% | 30.5% |
| Married couple, no children | 41.9% | 37.9% | 42.9% |
| Non Family Households | 1,697 | 12,198 | 43,255,233 |
| Single householder, w/children | 23.9% | 24.2% | 15.8% |
| Single householder, no children | 16.0% | 13.2% | 10.8% |