| | Waltham, MA | Hartford, CT | United States |
| Population | 64,655 | 121,562 | 329,725,481 |
| Female Population | 50.8% | 52.1% | 50.5% |
| Male Population | 49.2% | 47.9% | 49.5% |
| Median Age | 33.8 | 33 | 38.4 |
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| Population - 2010 | 60,912 | 125,349 | 309,327,143 |
| Population - 2000 | 59,226 | 121,578 | 285,036,114 |
| Population - 1990 | 57,878 | 139,739 | 251,960,433 |
| Pop. 2000 to Now | 4.8% | 0.0% | 15.6% |
| Pop. 1990 to Now | 7.2% | -13.0% | 30.8% |
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| Population Density | 5,075.8 | 6,994.8 | 93.4 |
| Land Area | 12.7 | 17.4 | 3,531,905.4 |
| Water Area | 1.0 | 0.7 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Waltham, MA | Hartford, CT | United States |
| Age 0 to 4 | 5.0% | 5.7% | 5.9% |
| Age 5 to 9 | 3.5% | 6.7% | 6.1% |
| Age 10 to 14 | 3.5% | 7.0% | 6.6% |
| Age 15 to 17 | 2.0% | 4.0% | 3.9% |
| Age 18 to 20 | 9.4% | 7.4% | 4.0% |
| Age 21 to 24 | 10.9% | 6.6% | 5.2% |
| Age 25 to 34 | 18.1% | 15.7% | 13.8% |
| Age 35 to 44 | 12.2% | 13.2% | 12.9% |
| Age 45 to 54 | 9.9% | 11.9% | 12.6% |
| Age 55 to 59 | 6.4% | 5.3% | 6.7% |
| Age 60 to 64 | 4.7% | 5.1% | 6.3% |
| Age 65 to 74 | 8.5% | 7.2% | 9.6% |
| Age 75 to 84 | 4.4% | 2.9% | 4.5% |
| Age 85 and over | 1.7% | 1.4% | 1.9% |
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| Race | | | |
| | Waltham, MA | Hartford, CT | United States |
| White | 63.8% | 14.7% | 59.4% |
| Black | 6.5% | 34.1% | 12.2% |
| Asian | 12.1% | 2.6% | 5.6% |
| Native American | 0.2% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.1% | 0.0% | 0.2% |
| Other | 0.3% | 0.6% | 0.4% |
| Two or More Races | 2.6% | 2.5% | 3.2% |
| Hispanic | 14.5% | 45.5% | 18.4% |
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| Marriage & Family | | | |
| | Waltham, MA | Hartford, CT | United States |
| Married Population | 38.2% | 27.4% | 49.8% |
| Currently Married | 36.9% | 24.9% | 48.0% |
| Married but Separated | 1.3% | 2.5% | 1.8% |
| Single Population | 61.8% | 72.6% | 50.2% |
| Never Married | 49.3% | 56.8% | 33.8% |
| Divorced | 7.3% | 11.4% | 10.8% |
| Widowed | 5.1% | 4.4% | 5.6% |
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| Household Size | 2.27 | 2.43 | 2.60 |
| Households | 24,943 | 46,879 | 124,010,992 |
| Family Households | 12,675 | 26,099 | 80,755,759 |
| Married couple, w/children | 29.5% | 20.1% | 30.5% |
| Married couple, no children | 46.8% | 19.3% | 42.9% |
| Non Family Households | 12,268 | 20,780 | 43,255,233 |
| Single householder, w/children | 9.2% | 37.6% | 15.8% |
| Single householder, no children | 14.4% | 23.1% | 10.8% |