| | St. Charles, SD | Gregory, SD | United States |
| Population | 8 | 1,317 | 329,725,481 |
| Female Population | 75.0% | 50.0% | 50.5% |
| Male Population | 25.0% | 50.0% | 49.5% |
| Median Age | 66.5 | 46.4 | 38.4 |
| space3 | | | |
| Population - 2010 | 22 | 1,292 | 309,327,143 |
| Population - 2000 | 19 | 1,342 | 285,036,114 |
| Population - 1990 | 10 | 1,384 | 251,960,433 |
| Pop. 2000 to Now | -36.8% | -6.9% | 15.6% |
| Pop. 1990 to Now | 20.0% | -9.7% | 30.8% |
| space4 | | | |
| Population Density | 40.6 | 765.3 | 93.4 |
| Land Area | 0.2 | 1.7 | 3,531,905.4 |
| Water Area | 0.0 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | St. Charles, SD | Gregory, SD | United States |
| Age 0 to 4 | 0.0% | 5.2% | 5.9% |
| Age 5 to 9 | 0.0% | 5.9% | 6.1% |
| Age 10 to 14 | 0.0% | 4.4% | 6.6% |
| Age 15 to 17 | 0.0% | 7.6% | 3.9% |
| Age 18 to 20 | 0.0% | 2.4% | 4.0% |
| Age 21 to 24 | 0.0% | 3.4% | 5.2% |
| Age 25 to 34 | 0.0% | 6.7% | 13.8% |
| Age 35 to 44 | 0.0% | 12.5% | 12.9% |
| Age 45 to 54 | 0.0% | 10.9% | 12.6% |
| Age 55 to 59 | 50.0% | 8.2% | 6.7% |
| Age 60 to 64 | 0.0% | 8.0% | 6.3% |
| Age 65 to 74 | 50.0% | 14.8% | 9.6% |
| Age 75 to 84 | 0.0% | 5.5% | 4.5% |
| Age 85 and over | 0.0% | 4.3% | 1.9% |
| space1 | | | |
| Race | | | |
| | St. Charles, SD | Gregory, SD | United States |
| White | 50.0% | 85.3% | 59.4% |
| Black | 0.0% | 3.8% | 12.2% |
| Asian | 0.0% | 0.0% | 5.6% |
| Native American | 50.0% | 1.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.0% | 0.4% |
| Two or More Races | 0.0% | 4.8% | 3.2% |
| Hispanic | 0.0% | 5.0% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | St. Charles, SD | Gregory, SD | United States |
| Married Population | 100.0% | 45.4% | 49.8% |
| Currently Married | 100.0% | 43.1% | 48.0% |
| Married but Separated | 0.0% | 2.2% | 1.8% |
| Single Population | 0.0% | 54.6% | 50.2% |
| Never Married | 0.0% | 36.6% | 33.8% |
| Divorced | 0.0% | 10.2% | 10.8% |
| Widowed | 0.0% | 7.8% | 5.6% |
| space5 | | | |
| Household Size | 1.33 | 2.24 | 2.60 |
| Households | 6 | 572 | 124,010,992 |
| Family Households | 2 | 272 | 80,755,759 |
| Married couple, w/children | 0.0% | 28.7% | 30.5% |
| Married couple, no children | 100.0% | 52.2% | 42.9% |
| Non Family Households | 4 | 300 | 43,255,233 |
| Single householder, w/children | 0.0% | 7.4% | 15.8% |
| Single householder, no children | 0.0% | 11.8% | 10.8% |