| | Eureka, CA | Rancho Cucamonga, CA | United States |
| Population | 26,763 | 173,946 | 329,725,481 |
| Female Population | 50.6% | 50.6% | 50.5% |
| Male Population | 49.5% | 49.4% | 49.5% |
| Median Age | 38.2 | 36.9 | 38.4 |
| space3 | | | |
| Population - 2010 | 27,201 | 166,361 | 309,327,143 |
| Population - 2000 | 26,128 | 127,743 | 285,036,114 |
| Population - 1990 | 27,025 | 101,409 | 251,960,433 |
| Pop. 2000 to Now | 2.0% | 40.0% | 15.6% |
| Pop. 1990 to Now | -1.4% | 76.4% | 30.8% |
| space4 | | | |
| Population Density | 2,852.0 | 4,336.0 | 93.4 |
| Land Area | 9.4 | 40.1 | 3,531,905.4 |
| Water Area | 5.1 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Eureka, CA | Rancho Cucamonga, CA | United States |
| Age 0 to 4 | 4.9% | 7.2% | 5.9% |
| Age 5 to 9 | 5.7% | 5.5% | 6.1% |
| Age 10 to 14 | 5.2% | 7.0% | 6.6% |
| Age 15 to 17 | 2.6% | 3.8% | 3.9% |
| Age 18 to 20 | 3.8% | 3.4% | 4.0% |
| Age 21 to 24 | 5.3% | 5.1% | 5.2% |
| Age 25 to 34 | 16.2% | 15.5% | 13.8% |
| Age 35 to 44 | 15.2% | 12.3% | 12.9% |
| Age 45 to 54 | 12.9% | 14.4% | 12.6% |
| Age 55 to 59 | 5.9% | 6.7% | 6.7% |
| Age 60 to 64 | 6.7% | 6.4% | 6.3% |
| Age 65 to 74 | 10.1% | 7.9% | 9.6% |
| Age 75 to 84 | 4.0% | 3.2% | 4.5% |
| Age 85 and over | 1.4% | 1.5% | 1.9% |
| space1 | | | |
| Race | | | |
| | Eureka, CA | Rancho Cucamonga, CA | United States |
| White | 65.0% | 33.6% | 59.4% |
| Black | 2.4% | 9.2% | 12.2% |
| Asian | 5.7% | 14.5% | 5.6% |
| Native American | 1.7% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.2% | 0.1% | 0.2% |
| Other | 0.3% | 0.6% | 0.4% |
| Two or More Races | 8.3% | 3.8% | 3.2% |
| Hispanic | 16.4% | 38.1% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Eureka, CA | Rancho Cucamonga, CA | United States |
| Married Population | 36.1% | 53.2% | 49.8% |
| Currently Married | 34.3% | 51.4% | 48.0% |
| Married but Separated | 1.8% | 1.9% | 1.8% |
| Single Population | 63.9% | 46.8% | 50.2% |
| Never Married | 41.8% | 32.4% | 33.8% |
| Divorced | 17.4% | 9.9% | 10.8% |
| Widowed | 4.8% | 4.5% | 5.6% |
| space5 | | | |
| Household Size | 2.37 | 3.03 | 2.60 |
| Households | 10,808 | 56,380 | 124,010,992 |
| Family Households | 5,213 | 43,212 | 80,755,759 |
| Married couple, w/children | 27.6% | 32.7% | 30.5% |
| Married couple, no children | 39.2% | 40.7% | 42.9% |
| Non Family Households | 5,595 | 13,168 | 43,255,233 |
| Single householder, w/children | 19.6% | 13.6% | 15.8% |
| Single householder, no children | 13.6% | 13.0% | 10.8% |