| | Belmont, MA | Arlington, MA | United States |
| Population | 27,056 | 46,045 | 329,725,481 |
| Female Population | 52.8% | 53.3% | 50.5% |
| Male Population | 47.2% | 46.7% | 49.5% |
| Median Age | 41.5 | 39.7 | 38.4 |
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| Population - 2010 | 24,700 | 42,946 | 309,327,143 |
| Population - 2000 | 24,194 | 42,389 | 285,036,114 |
| Population - 1990 | 24,720 | 44,630 | 251,960,433 |
| Pop. 2000 to Now | 6.9% | 6.6% | 15.6% |
| Pop. 1990 to Now | 4.7% | 1.2% | 30.8% |
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| Population Density | 5,817.2 | 8,947.7 | 93.4 |
| Land Area | 4.7 | 5.1 | 3,531,905.4 |
| Water Area | 0.1 | 0.3 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Belmont, MA | Arlington, MA | United States |
| Age 0 to 4 | 6.3% | 6.5% | 5.9% |
| Age 5 to 9 | 7.3% | 5.6% | 6.1% |
| Age 10 to 14 | 6.1% | 6.1% | 6.6% |
| Age 15 to 17 | 5.0% | 3.2% | 3.9% |
| Age 18 to 20 | 2.3% | 1.6% | 4.0% |
| Age 21 to 24 | 2.9% | 3.4% | 5.2% |
| Age 25 to 34 | 9.8% | 16.0% | 13.8% |
| Age 35 to 44 | 15.3% | 15.1% | 12.9% |
| Age 45 to 54 | 15.1% | 13.5% | 12.6% |
| Age 55 to 59 | 7.0% | 6.4% | 6.7% |
| Age 60 to 64 | 5.6% | 6.6% | 6.3% |
| Age 65 to 74 | 10.0% | 9.5% | 9.6% |
| Age 75 to 84 | 5.0% | 4.6% | 4.5% |
| Age 85 and over | 2.3% | 1.9% | 1.9% |
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| Race | | | |
| | Belmont, MA | Arlington, MA | United States |
| White | 72.5% | 72.9% | 59.4% |
| Black | 1.2% | 3.3% | 12.2% |
| Asian | 18.2% | 13.7% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.1% | 0.2% | 0.4% |
| Two or More Races | 3.9% | 4.7% | 3.2% |
| Hispanic | 4.2% | 5.0% | 18.4% |
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| Marriage & Family | | | |
| | Belmont, MA | Arlington, MA | United States |
| Married Population | 64.4% | 57.3% | 49.8% |
| Currently Married | 63.6% | 55.8% | 48.0% |
| Married but Separated | 0.9% | 1.5% | 1.8% |
| Single Population | 35.6% | 42.7% | 50.2% |
| Never Married | 24.7% | 31.4% | 33.8% |
| Divorced | 5.6% | 7.6% | 10.8% |
| Widowed | 5.3% | 3.7% | 5.6% |
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| Household Size | 2.58 | 2.37 | 2.60 |
| Households | 10,393 | 19,308 | 124,010,992 |
| Family Households | 7,556 | 11,982 | 80,755,759 |
| Married couple, w/children | 45.1% | 40.5% | 30.5% |
| Married couple, no children | 41.5% | 41.4% | 42.9% |
| Non Family Households | 2,837 | 7,326 | 43,255,233 |
| Single householder, w/children | 7.7% | 8.7% | 15.8% |
| Single householder, no children | 5.8% | 9.3% | 10.8% |