| | Boulder, CO | Castro Valley, CA | United States |
| Population | 104,930 | 66,324 | 329,725,481 |
| Female Population | 48.5% | 51.4% | 50.5% |
| Male Population | 51.5% | 48.6% | 49.5% |
| Median Age | 28.9 | 42.4 | 38.4 |
| space3 | | | |
| Population - 2010 | 97,724 | 60,625 | 309,327,143 |
| Population - 2000 | 94,673 | 57,292 | 285,036,114 |
| Population - 1990 | 83,312 | 48,619 | 251,960,433 |
| Pop. 2000 to Now | 13.7% | 13.0% | 15.6% |
| Pop. 1990 to Now | 29.2% | 33.1% | 30.8% |
| space4 | | | |
| Population Density | 3,991.4 | 3,997.8 | 93.4 |
| Land Area | 26.3 | 16.6 | 3,531,905.4 |
| Water Area | 1.0 | 0.2 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Boulder, CO | Castro Valley, CA | United States |
| Age 0 to 4 | 2.7% | 5.1% | 5.9% |
| Age 5 to 9 | 3.7% | 6.1% | 6.1% |
| Age 10 to 14 | 3.6% | 5.5% | 6.6% |
| Age 15 to 17 | 2.5% | 4.3% | 3.9% |
| Age 18 to 20 | 16.2% | 1.9% | 4.0% |
| Age 21 to 24 | 13.8% | 3.5% | 5.2% |
| Age 25 to 34 | 16.3% | 12.2% | 13.8% |
| Age 35 to 44 | 10.3% | 14.4% | 12.9% |
| Age 45 to 54 | 10.3% | 14.3% | 12.6% |
| Age 55 to 59 | 4.5% | 7.8% | 6.7% |
| Age 60 to 64 | 4.2% | 7.5% | 6.3% |
| Age 65 to 74 | 7.0% | 10.4% | 9.6% |
| Age 75 to 84 | 3.1% | 4.4% | 4.5% |
| Age 85 and over | 1.8% | 2.4% | 1.9% |
| space1 | | | |
| Race | | | |
| | Boulder, CO | Castro Valley, CA | United States |
| White | 78.3% | 36.7% | 59.4% |
| Black | 1.0% | 9.3% | 12.2% |
| Asian | 5.9% | 31.2% | 5.6% |
| Native American | 0.2% | 0.5% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.2% | 0.2% |
| Other | 0.3% | 0.3% | 0.4% |
| Two or More Races | 3.5% | 4.8% | 3.2% |
| Hispanic | 10.6% | 16.9% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Boulder, CO | Castro Valley, CA | United States |
| Married Population | 31.9% | 55.8% | 49.8% |
| Currently Married | 31.2% | 54.5% | 48.0% |
| Married but Separated | 0.7% | 1.3% | 1.8% |
| Single Population | 68.1% | 44.2% | 50.2% |
| Never Married | 56.1% | 29.8% | 33.8% |
| Divorced | 9.5% | 8.4% | 10.8% |
| Widowed | 2.5% | 6.0% | 5.6% |
| space5 | | | |
| Household Size | 2.21 | 2.90 | 2.60 |
| Households | 42,610 | 22,467 | 124,010,992 |
| Family Households | 17,341 | 16,692 | 80,755,759 |
| Married couple, w/children | 33.3% | 35.0% | 30.5% |
| Married couple, no children | 47.4% | 45.3% | 42.9% |
| Non Family Households | 25,269 | 5,775 | 43,255,233 |
| Single householder, w/children | 10.8% | 11.2% | 15.8% |
| Single householder, no children | 8.5% | 8.5% | 10.8% |