| | Eureka, UT | Placentia, CA | United States |
| Population | 596 | 51,951 | 329,725,481 |
| Female Population | 50.3% | 50.5% | 50.5% |
| Male Population | 49.7% | 49.5% | 49.5% |
| Median Age | 46.5 | 36.9 | 38.4 |
| space3 | | | |
| Population - 2010 | 669 | 50,937 | 309,327,143 |
| Population - 2000 | 766 | 46,488 | 285,036,114 |
| Population - 1990 | 562 | 41,259 | 251,960,433 |
| Pop. 2000 to Now | -8.4% | 10.8% | 15.6% |
| Pop. 1990 to Now | 24.9% | 24.9% | 30.8% |
| space4 | | | |
| Population Density | 401.6 | 7,854.7 | 93.4 |
| Land Area | 1.5 | 6.6 | 3,531,905.4 |
| Water Area | 0.0 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Eureka, UT | Placentia, CA | United States |
| Age 0 to 4 | 6.4% | 5.6% | 5.9% |
| Age 5 to 9 | 3.7% | 6.2% | 6.1% |
| Age 10 to 14 | 7.6% | 7.6% | 6.6% |
| Age 15 to 17 | 8.9% | 4.8% | 3.9% |
| Age 18 to 20 | 5.4% | 3.2% | 4.0% |
| Age 21 to 24 | 5.4% | 6.3% | 5.2% |
| Age 25 to 34 | 7.9% | 13.7% | 13.8% |
| Age 35 to 44 | 4.0% | 13.0% | 12.9% |
| Age 45 to 54 | 13.6% | 13.8% | 12.6% |
| Age 55 to 59 | 7.9% | 6.2% | 6.7% |
| Age 60 to 64 | 8.1% | 6.0% | 6.3% |
| Age 65 to 74 | 12.9% | 7.8% | 9.6% |
| Age 75 to 84 | 4.7% | 4.0% | 4.5% |
| Age 85 and over | 3.7% | 1.8% | 1.9% |
| space1 | | | |
| Race | | | |
| | Eureka, UT | Placentia, CA | United States |
| White | 89.1% | 38.2% | 59.4% |
| Black | 0.0% | 2.5% | 12.2% |
| Asian | 0.0% | 16.0% | 5.6% |
| Native American | 0.5% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.2% | 0.2% |
| Other | 0.0% | 0.5% | 0.4% |
| Two or More Races | 0.3% | 3.5% | 3.2% |
| Hispanic | 10.1% | 39.1% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Eureka, UT | Placentia, CA | United States |
| Married Population | 47.9% | 52.6% | 49.8% |
| Currently Married | 47.9% | 50.7% | 48.0% |
| Married but Separated | 0.0% | 1.8% | 1.8% |
| Single Population | 52.1% | 47.5% | 50.2% |
| Never Married | 27.5% | 34.1% | 33.8% |
| Divorced | 16.1% | 8.6% | 10.8% |
| Widowed | 8.6% | 4.8% | 5.6% |
| space5 | | | |
| Household Size | 2.80 | 3.14 | 2.60 |
| Households | 213 | 16,371 | 124,010,992 |
| Family Households | 145 | 12,623 | 80,755,759 |
| Married couple, w/children | 17.2% | 37.3% | 30.5% |
| Married couple, no children | 55.2% | 38.5% | 42.9% |
| Non Family Households | 68 | 3,748 | 43,255,233 |
| Single householder, w/children | 22.1% | 11.1% | 15.8% |
| Single householder, no children | 5.5% | 13.1% | 10.8% |