| | Eureka, MT | Rapid City, SD | United States |
| Population | 1,447 | 73,942 | 329,725,481 |
| Female Population | 52.7% | 49.6% | 50.5% |
| Male Population | 47.3% | 50.4% | 49.5% |
| Median Age | 42.4 | 38.1 | 38.4 |
| space3 | | | |
| Population - 2010 | 1,403 | 69,138 | 309,327,143 |
| Population - 2000 | 1,017 | 59,607 | 285,036,114 |
| Population - 1990 | 1,043 | 54,523 | 251,960,433 |
| Pop. 2000 to Now | 42.1% | 32.3% | 15.6% |
| Pop. 1990 to Now | 38.5% | 44.6% | 30.8% |
| space4 | | | |
| Population Density | 1,400.8 | 1,356.1 | 93.4 |
| Land Area | 1.0 | 54.5 | 3,531,905.4 |
| Water Area | 0.0 | 0.1 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Eureka, MT | Rapid City, SD | United States |
| Age 0 to 4 | 4.8% | 6.7% | 5.9% |
| Age 5 to 9 | 4.4% | 6.1% | 6.1% |
| Age 10 to 14 | 7.0% | 6.4% | 6.6% |
| Age 15 to 17 | 7.3% | 3.6% | 3.9% |
| Age 18 to 20 | 2.6% | 3.7% | 4.0% |
| Age 21 to 24 | 1.0% | 5.2% | 5.2% |
| Age 25 to 34 | 14.9% | 14.0% | 13.8% |
| Age 35 to 44 | 11.7% | 12.2% | 12.9% |
| Age 45 to 54 | 12.3% | 10.6% | 12.6% |
| Age 55 to 59 | 9.4% | 6.1% | 6.7% |
| Age 60 to 64 | 7.4% | 6.5% | 6.3% |
| Age 65 to 74 | 12.2% | 11.5% | 9.6% |
| Age 75 to 84 | 2.1% | 5.1% | 4.5% |
| Age 85 and over | 3.0% | 2.3% | 1.9% |
| space1 | | | |
| Race | | | |
| | Eureka, MT | Rapid City, SD | United States |
| White | 89.1% | 77.0% | 59.4% |
| Black | 0.0% | 1.4% | 12.2% |
| Asian | 0.0% | 1.3% | 5.6% |
| Native American | 3.0% | 8.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.1% | 0.2% |
| Other | 0.0% | 0.6% | 0.4% |
| Two or More Races | 1.2% | 5.6% | 3.2% |
| Hispanic | 6.6% | 5.9% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Eureka, MT | Rapid City, SD | United States |
| Married Population | 44.9% | 45.9% | 49.8% |
| Currently Married | 43.9% | 44.6% | 48.0% |
| Married but Separated | 1.0% | 1.2% | 1.8% |
| Single Population | 55.1% | 54.1% | 50.2% |
| Never Married | 34.4% | 32.5% | 33.8% |
| Divorced | 14.8% | 15.0% | 10.8% |
| Widowed | 5.9% | 6.6% | 5.6% |
| space5 | | | |
| Household Size | 2.46 | 2.29 | 2.60 |
| Households | 575 | 30,737 | 124,010,992 |
| Family Households | 279 | 16,970 | 80,755,759 |
| Married couple, w/children | 26.2% | 27.2% | 30.5% |
| Married couple, no children | 58.1% | 47.9% | 42.9% |
| Non Family Households | 296 | 13,767 | 43,255,233 |
| Single householder, w/children | 11.5% | 16.8% | 15.8% |
| Single householder, no children | 4.3% | 8.1% | 10.8% |