| | Shelby, MT | Sundance, WY | United States |
| Population | 3,139 | 1,361 | 329,725,481 |
| Female Population | 39.9% | 57.2% | 50.5% |
| Male Population | 60.2% | 42.8% | 49.5% |
| Median Age | 41.9 | 30.1 | 38.4 |
| space3 | | | |
| Population - 2010 | 3,410 | 1,191 | 309,327,143 |
| Population - 2000 | 3,216 | 1,161 | 285,036,114 |
| Population - 1990 | 2,763 | 1,139 | 251,960,433 |
| Pop. 2000 to Now | -6.9% | 11.6% | 15.6% |
| Pop. 1990 to Now | 8.4% | 13.8% | 30.8% |
| space4 | | | |
| Population Density | 542.9 | 446.1 | 93.4 |
| Land Area | 5.8 | 3.1 | 3,531,905.4 |
| Water Area | 0.2 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Shelby, MT | Sundance, WY | United States |
| Age 0 to 4 | 4.6% | 12.5% | 5.9% |
| Age 5 to 9 | 5.2% | 5.2% | 6.1% |
| Age 10 to 14 | 3.0% | 3.0% | 6.6% |
| Age 15 to 17 | 4.5% | 1.8% | 3.9% |
| Age 18 to 20 | 0.7% | 3.2% | 4.0% |
| Age 21 to 24 | 6.4% | 4.6% | 5.2% |
| Age 25 to 34 | 16.8% | 28.8% | 13.8% |
| Age 35 to 44 | 14.2% | 5.2% | 12.9% |
| Age 45 to 54 | 10.5% | 6.1% | 12.6% |
| Age 55 to 59 | 11.1% | 6.0% | 6.7% |
| Age 60 to 64 | 6.2% | 6.2% | 6.3% |
| Age 65 to 74 | 10.3% | 10.9% | 9.6% |
| Age 75 to 84 | 5.0% | 4.8% | 4.5% |
| Age 85 and over | 1.5% | 1.8% | 1.9% |
| space1 | | | |
| Race | | | |
| | Shelby, MT | Sundance, WY | United States |
| White | 83.2% | 88.8% | 59.4% |
| Black | 4.7% | 0.0% | 12.2% |
| Asian | 0.2% | 0.0% | 5.6% |
| Native American | 7.8% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.0% | 0.4% |
| Two or More Races | 1.8% | 5.2% | 3.2% |
| Hispanic | 2.3% | 6.0% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Shelby, MT | Sundance, WY | United States |
| Married Population | 44.9% | 56.2% | 49.8% |
| Currently Married | 43.6% | 56.2% | 48.0% |
| Married but Separated | 1.4% | 0.0% | 1.8% |
| Single Population | 55.1% | 43.8% | 50.2% |
| Never Married | 34.3% | 23.0% | 33.8% |
| Divorced | 14.8% | 15.9% | 10.8% |
| Widowed | 6.0% | 4.9% | 5.6% |
| space5 | | | |
| Household Size | 2.18 | 2.31 | 2.60 |
| Households | 1,180 | 566 | 124,010,992 |
| Family Households | 645 | 377 | 80,755,759 |
| Married couple, w/children | 30.2% | 42.2% | 30.5% |
| Married couple, no children | 56.1% | 43.0% | 42.9% |
| Non Family Households | 535 | 189 | 43,255,233 |
| Single householder, w/children | 10.1% | 9.3% | 15.8% |
| Single householder, no children | 3.6% | 5.6% | 10.8% |