| | Belfast, ME | Riverside, CA | United States |
| Population | 6,890 | 314,858 | 329,725,481 |
| Female Population | 56.0% | 50.0% | 50.5% |
| Male Population | 44.0% | 50.0% | 49.5% |
| Median Age | 60.1 | 31.9 | 38.4 |
| space3 | | | |
| Population - 2010 | 6,670 | 305,417 | 309,327,143 |
| Population - 2000 | 6,381 | 255,166 | 285,036,114 |
| Population - 1990 | 6,355 | 226,505 | 251,960,433 |
| Pop. 2000 to Now | 5.1% | 29.6% | 15.6% |
| Pop. 1990 to Now | 5.6% | 46.0% | 30.8% |
| space4 | | | |
| Population Density | 202.4 | 3,875.0 | 93.4 |
| Land Area | 34.0 | 81.3 | 3,531,905.4 |
| Water Area | 4.3 | 0.3 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Belfast, ME | Riverside, CA | United States |
| Age 0 to 4 | 3.4% | 6.0% | 5.9% |
| Age 5 to 9 | 3.8% | 6.2% | 6.1% |
| Age 10 to 14 | 5.8% | 7.6% | 6.6% |
| Age 15 to 17 | 1.8% | 4.4% | 3.9% |
| Age 18 to 20 | 2.1% | 7.0% | 4.0% |
| Age 21 to 24 | 3.1% | 7.1% | 5.2% |
| Age 25 to 34 | 9.7% | 15.9% | 13.8% |
| Age 35 to 44 | 8.9% | 13.4% | 12.9% |
| Age 45 to 54 | 8.1% | 11.8% | 12.6% |
| Age 55 to 59 | 3.1% | 5.3% | 6.7% |
| Age 60 to 64 | 14.7% | 4.6% | 6.3% |
| Age 65 to 74 | 24.3% | 6.6% | 9.6% |
| Age 75 to 84 | 8.7% | 2.8% | 4.5% |
| Age 85 and over | 2.6% | 1.2% | 1.9% |
| space1 | | | |
| Race | | | |
| | Belfast, ME | Riverside, CA | United States |
| White | 91.0% | 27.6% | 59.4% |
| Black | 1.2% | 5.4% | 12.2% |
| Asian | 0.4% | 8.5% | 5.6% |
| Native American | 0.4% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.3% | 0.2% |
| Other | 2.8% | 0.3% | 0.4% |
| Two or More Races | 3.6% | 2.8% | 3.2% |
| Hispanic | 0.5% | 54.9% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Belfast, ME | Riverside, CA | United States |
| Married Population | 54.1% | 44.4% | 49.8% |
| Currently Married | 53.6% | 42.3% | 48.0% |
| Married but Separated | 0.5% | 2.1% | 1.8% |
| Single Population | 45.9% | 55.6% | 50.2% |
| Never Married | 22.9% | 42.9% | 33.8% |
| Divorced | 16.6% | 8.4% | 10.8% |
| Widowed | 6.4% | 4.3% | 5.6% |
| space5 | | | |
| Household Size | 2.03 | 3.38 | 2.60 |
| Households | 3,327 | 88,340 | 124,010,992 |
| Family Households | 2,074 | 65,020 | 80,755,759 |
| Married couple, w/children | 13.7% | 36.3% | 30.5% |
| Married couple, no children | 60.4% | 32.9% | 42.9% |
| Non Family Households | 1,253 | 23,320 | 43,255,233 |
| Single householder, w/children | 18.1% | 17.7% | 15.8% |
| Single householder, no children | 7.8% | 13.1% | 10.8% |