| | Sidney, OH | Watertown, SD | United States |
| Population | 20,559 | 22,577 | 329,725,481 |
| Female Population | 51.5% | 49.7% | 50.5% |
| Male Population | 48.5% | 50.3% | 49.5% |
| Median Age | 39 | 38.2 | 38.4 |
| space3 | | | |
| Population - 2010 | 21,158 | 21,520 | 309,327,143 |
| Population - 2000 | 20,211 | 20,237 | 285,036,114 |
| Population - 1990 | 18,710 | 17,592 | 251,960,433 |
| Pop. 2000 to Now | 0.5% | 10.2% | 15.6% |
| Pop. 1990 to Now | 8.5% | 26.8% | 30.8% |
| space4 | | | |
| Population Density | 1,718.1 | 1,252.3 | 93.4 |
| Land Area | 12.0 | 18.0 | 3,531,905.4 |
| Water Area | 0.1 | 8.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Sidney, OH | Watertown, SD | United States |
| Age 0 to 4 | 6.2% | 5.9% | 5.9% |
| Age 5 to 9 | 5.2% | 6.9% | 6.1% |
| Age 10 to 14 | 7.7% | 6.7% | 6.6% |
| Age 15 to 17 | 4.7% | 3.9% | 3.9% |
| Age 18 to 20 | 4.2% | 4.2% | 4.0% |
| Age 21 to 24 | 5.8% | 4.8% | 5.2% |
| Age 25 to 34 | 10.9% | 13.6% | 13.8% |
| Age 35 to 44 | 11.6% | 12.7% | 12.9% |
| Age 45 to 54 | 13.8% | 10.8% | 12.6% |
| Age 55 to 59 | 7.7% | 6.4% | 6.7% |
| Age 60 to 64 | 5.8% | 6.2% | 6.3% |
| Age 65 to 74 | 9.8% | 9.5% | 9.6% |
| Age 75 to 84 | 5.5% | 5.3% | 4.5% |
| Age 85 and over | 1.0% | 3.0% | 1.9% |
| space1 | | | |
| Race | | | |
| | Sidney, OH | Watertown, SD | United States |
| White | 87.2% | 89.7% | 59.4% |
| Black | 4.2% | 1.1% | 12.2% |
| Asian | 1.3% | 0.7% | 5.6% |
| Native American | 0.3% | 2.5% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.5% | 0.4% |
| Two or More Races | 4.2% | 1.9% | 3.2% |
| Hispanic | 2.9% | 3.6% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Sidney, OH | Watertown, SD | United States |
| Married Population | 45.6% | 51.2% | 49.8% |
| Currently Married | 42.7% | 49.7% | 48.0% |
| Married but Separated | 2.9% | 1.5% | 1.8% |
| Single Population | 54.4% | 48.8% | 50.2% |
| Never Married | 33.0% | 30.7% | 33.8% |
| Divorced | 15.2% | 10.7% | 10.8% |
| Widowed | 6.2% | 7.4% | 5.6% |
| space5 | | | |
| Household Size | 2.42 | 2.26 | 2.60 |
| Households | 8,393 | 9,725 | 124,010,992 |
| Family Households | 5,039 | 5,562 | 80,755,759 |
| Married couple, w/children | 23.6% | 34.2% | 30.5% |
| Married couple, no children | 43.5% | 43.4% | 42.9% |
| Non Family Households | 3,354 | 4,163 | 43,255,233 |
| Single householder, w/children | 23.2% | 12.4% | 15.8% |
| Single householder, no children | 9.8% | 10.0% | 10.8% |