| | Spearfish, SD | Post Falls, ID | United States |
| Population | 12,031 | 38,162 | 329,725,481 |
| Female Population | 51.6% | 52.1% | 50.5% |
| Male Population | 48.4% | 47.9% | 49.5% |
| Median Age | 36.8 | 34.1 | 38.4 |
| space3 | | | |
| Population - 2010 | 10,529 | 27,950 | 309,327,143 |
| Population - 2000 | 8,606 | 17,247 | 285,036,114 |
| Population - 1990 | 6,966 | 7,349 | 251,960,433 |
| Pop. 2000 to Now | 38.3% | 125.7% | 15.6% |
| Pop. 1990 to Now | 70.8% | 429.8% | 30.8% |
| space4 | | | |
| Population Density | 732.1 | 2,613.1 | 93.4 |
| Land Area | 16.4 | 14.6 | 3,531,905.4 |
| Water Area | 0.0 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Spearfish, SD | Post Falls, ID | United States |
| Age 0 to 4 | 5.1% | 8.3% | 5.9% |
| Age 5 to 9 | 3.7% | 7.3% | 6.1% |
| Age 10 to 14 | 4.5% | 8.1% | 6.6% |
| Age 15 to 17 | 3.4% | 5.4% | 3.9% |
| Age 18 to 20 | 10.7% | 3.6% | 4.0% |
| Age 21 to 24 | 8.3% | 4.0% | 5.2% |
| Age 25 to 34 | 12.4% | 14.6% | 13.8% |
| Age 35 to 44 | 10.7% | 14.6% | 12.9% |
| Age 45 to 54 | 7.4% | 9.5% | 12.6% |
| Age 55 to 59 | 3.9% | 6.4% | 6.7% |
| Age 60 to 64 | 7.8% | 5.6% | 6.3% |
| Age 65 to 74 | 12.0% | 8.0% | 9.6% |
| Age 75 to 84 | 6.8% | 3.3% | 4.5% |
| Age 85 and over | 3.2% | 1.4% | 1.9% |
| space1 | | | |
| Race | | | |
| | Spearfish, SD | Post Falls, ID | United States |
| White | 89.7% | 87.2% | 59.4% |
| Black | 0.7% | 0.2% | 12.2% |
| Asian | 0.5% | 0.6% | 5.6% |
| Native American | 3.0% | 0.4% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.1% | 0.2% |
| Other | 0.0% | 0.0% | 0.4% |
| Two or More Races | 4.3% | 5.2% | 3.2% |
| Hispanic | 1.8% | 6.4% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Spearfish, SD | Post Falls, ID | United States |
| Married Population | 42.4% | 52.1% | 49.8% |
| Currently Married | 41.1% | 50.6% | 48.0% |
| Married but Separated | 1.3% | 1.5% | 1.8% |
| Single Population | 57.6% | 47.9% | 50.2% |
| Never Married | 38.5% | 28.0% | 33.8% |
| Divorced | 12.6% | 14.3% | 10.8% |
| Widowed | 6.5% | 5.7% | 5.6% |
| space5 | | | |
| Household Size | 2.14 | 2.84 | 2.60 |
| Households | 5,232 | 13,352 | 124,010,992 |
| Family Households | 2,400 | 9,662 | 80,755,759 |
| Married couple, w/children | 27.8% | 34.6% | 30.5% |
| Married couple, no children | 57.0% | 38.1% | 42.9% |
| Non Family Households | 2,832 | 3,690 | 43,255,233 |
| Single householder, w/children | 11.3% | 15.9% | 15.8% |
| Single householder, no children | 4.0% | 11.4% | 10.8% |