| | Belmont, CA | Napa, CA | United States |
| Population | 28,109 | 79,700 | 329,725,481 |
| Female Population | 49.1% | 50.3% | 50.5% |
| Male Population | 50.9% | 49.7% | 49.5% |
| Median Age | 40.2 | 39.9 | 38.4 |
| space3 | | | |
| Population - 2010 | 25,933 | 77,187 | 309,327,143 |
| Population - 2000 | 25,123 | 72,585 | 285,036,114 |
| Population - 1990 | 24,127 | 61,842 | 251,960,433 |
| Pop. 2000 to Now | 5.6% | 6.1% | 15.6% |
| Pop. 1990 to Now | 10.0% | 24.6% | 30.8% |
| space4 | | | |
| Population Density | 6,065.8 | 4,439.9 | 93.4 |
| Land Area | 4.6 | 18.0 | 3,531,905.4 |
| Water Area | 0.0 | 0.3 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Belmont, CA | Napa, CA | United States |
| Age 0 to 4 | 6.0% | 5.4% | 5.9% |
| Age 5 to 9 | 6.3% | 5.4% | 6.1% |
| Age 10 to 14 | 6.5% | 6.8% | 6.6% |
| Age 15 to 17 | 4.7% | 4.0% | 3.9% |
| Age 18 to 20 | 2.5% | 3.4% | 4.0% |
| Age 21 to 24 | 2.4% | 4.7% | 5.2% |
| Age 25 to 34 | 12.7% | 13.6% | 13.8% |
| Age 35 to 44 | 16.5% | 13.5% | 12.9% |
| Age 45 to 54 | 14.8% | 13.2% | 12.6% |
| Age 55 to 59 | 7.3% | 6.5% | 6.7% |
| Age 60 to 64 | 5.6% | 6.1% | 6.3% |
| Age 65 to 74 | 8.1% | 10.4% | 9.6% |
| Age 75 to 84 | 4.0% | 5.0% | 4.5% |
| Age 85 and over | 2.6% | 2.1% | 1.9% |
| space1 | | | |
| Race | | | |
| | Belmont, CA | Napa, CA | United States |
| White | 49.1% | 53.0% | 59.4% |
| Black | 1.1% | 0.6% | 12.2% |
| Asian | 29.9% | 2.9% | 5.6% |
| Native American | 0.2% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 1.4% | 0.1% | 0.2% |
| Other | 1.1% | 0.4% | 0.4% |
| Two or More Races | 5.3% | 3.2% | 3.2% |
| Hispanic | 12.1% | 39.6% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Belmont, CA | Napa, CA | United States |
| Married Population | 62.1% | 51.2% | 49.8% |
| Currently Married | 60.8% | 49.8% | 48.0% |
| Married but Separated | 1.2% | 1.4% | 1.8% |
| Single Population | 37.9% | 48.8% | 50.2% |
| Never Married | 25.6% | 32.2% | 33.8% |
| Divorced | 8.6% | 11.1% | 10.8% |
| Widowed | 3.6% | 5.5% | 5.6% |
| space5 | | | |
| Household Size | 2.51 | 2.71 | 2.60 |
| Households | 10,961 | 29,099 | 124,010,992 |
| Family Households | 7,662 | 19,393 | 80,755,759 |
| Married couple, w/children | 44.3% | 32.1% | 30.5% |
| Married couple, no children | 43.9% | 44.4% | 42.9% |
| Non Family Households | 3,299 | 9,706 | 43,255,233 |
| Single householder, w/children | 7.6% | 13.6% | 15.8% |
| Single householder, no children | 4.3% | 9.9% | 10.8% |