| | Belmont, NY | Rockville, MD | United States |
| Population | 866 | 67,095 | 329,725,481 |
| Female Population | 51.7% | 51.9% | 50.5% |
| Male Population | 48.3% | 48.1% | 49.5% |
| Median Age | 38.3 | 39.2 | 38.4 |
| space3 | | | |
| Population - 2010 | 970 | 61,542 | 309,327,143 |
| Population - 2000 | 952 | 47,388 | 285,036,114 |
| Population - 1990 | 1,006 | 44,835 | 251,960,433 |
| Pop. 2000 to Now | -6.2% | 46.7% | 15.6% |
| Pop. 1990 to Now | -11.2% | 55.0% | 30.8% |
| space4 | | | |
| Population Density | 869.5 | 4,936.0 | 93.4 |
| Land Area | 1.0 | 13.6 | 3,531,905.4 |
| Water Area | 0.0 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Belmont, NY | Rockville, MD | United States |
| Age 0 to 4 | 7.3% | 5.4% | 5.9% |
| Age 5 to 9 | 10.6% | 6.0% | 6.1% |
| Age 10 to 14 | 5.3% | 6.5% | 6.6% |
| Age 15 to 17 | 4.9% | 3.2% | 3.9% |
| Age 18 to 20 | 2.2% | 2.7% | 4.0% |
| Age 21 to 24 | 2.0% | 4.5% | 5.2% |
| Age 25 to 34 | 16.3% | 14.5% | 13.8% |
| Age 35 to 44 | 6.2% | 15.2% | 12.9% |
| Age 45 to 54 | 14.7% | 12.7% | 12.6% |
| Age 55 to 59 | 3.8% | 6.4% | 6.7% |
| Age 60 to 64 | 7.6% | 5.6% | 6.3% |
| Age 65 to 74 | 10.6% | 9.0% | 9.6% |
| Age 75 to 84 | 6.0% | 5.8% | 4.5% |
| Age 85 and over | 2.5% | 2.6% | 1.9% |
| space1 | | | |
| Race | | | |
| | Belmont, NY | Rockville, MD | United States |
| White | 96.2% | 47.7% | 59.4% |
| Black | 2.3% | 10.1% | 12.2% |
| Asian | 0.0% | 20.3% | 5.6% |
| Native American | 0.0% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 1.1% | 0.4% |
| Two or More Races | 0.3% | 4.7% | 3.2% |
| Hispanic | 1.2% | 16.0% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Belmont, NY | Rockville, MD | United States |
| Married Population | 43.6% | 53.6% | 49.8% |
| Currently Married | 41.5% | 52.2% | 48.0% |
| Married but Separated | 2.1% | 1.4% | 1.8% |
| Single Population | 56.4% | 46.4% | 50.2% |
| Never Married | 32.9% | 31.6% | 33.8% |
| Divorced | 18.9% | 9.4% | 10.8% |
| Widowed | 4.5% | 5.3% | 5.6% |
| space5 | | | |
| Household Size | 2.38 | 2.50 | 2.60 |
| Households | 356 | 26,536 | 124,010,992 |
| Family Households | 181 | 17,039 | 80,755,759 |
| Married couple, w/children | 33.1% | 33.7% | 30.5% |
| Married couple, no children | 39.8% | 41.4% | 42.9% |
| Non Family Households | 175 | 9,497 | 43,255,233 |
| Single householder, w/children | 27.1% | 12.7% | 15.8% |
| Single householder, no children | 0.0% | 12.2% | 10.8% |