| | Belleville, NJ | Westwood, NJ | United States |
| Population | 37,914 | 11,267 | 329,725,481 |
| Female Population | 51.9% | 51.4% | 50.5% |
| Male Population | 48.1% | 48.7% | 49.5% |
| Median Age | 41.3 | 42.4 | 38.4 |
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| Population - 2010 | 35,822 | 10,956 | 309,327,143 |
| Population - 2000 | 35,928 | 10,999 | 285,036,114 |
| Population - 1990 | 34,213 | 10,446 | 251,960,433 |
| Pop. 2000 to Now | 1.4% | 0.2% | 15.6% |
| Pop. 1990 to Now | 6.5% | 5.5% | 30.8% |
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| Population Density | 11,351.5 | 4,987.6 | 93.4 |
| Land Area | 3.3 | 2.3 | 3,531,905.4 |
| Water Area | 0.1 | 0.0 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Belleville, NJ | Westwood, NJ | United States |
| Age 0 to 4 | 5.5% | 3.3% | 5.9% |
| Age 5 to 9 | 5.5% | 7.3% | 6.1% |
| Age 10 to 14 | 3.9% | 7.1% | 6.6% |
| Age 15 to 17 | 3.8% | 2.9% | 3.9% |
| Age 18 to 20 | 3.9% | 2.2% | 4.0% |
| Age 21 to 24 | 5.9% | 4.3% | 5.2% |
| Age 25 to 34 | 13.8% | 9.5% | 13.8% |
| Age 35 to 44 | 12.0% | 17.1% | 12.9% |
| Age 45 to 54 | 17.3% | 13.9% | 12.6% |
| Age 55 to 59 | 6.9% | 5.3% | 6.7% |
| Age 60 to 64 | 6.8% | 4.7% | 6.3% |
| Age 65 to 74 | 8.5% | 10.5% | 9.6% |
| Age 75 to 84 | 4.0% | 8.7% | 4.5% |
| Age 85 and over | 2.3% | 3.2% | 1.9% |
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| Race | | | |
| | Belleville, NJ | Westwood, NJ | United States |
| White | 30.3% | 63.7% | 59.4% |
| Black | 7.3% | 1.6% | 12.2% |
| Asian | 10.5% | 9.6% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 1.3% | 1.2% | 0.4% |
| Two or More Races | 2.0% | 3.8% | 3.2% |
| Hispanic | 48.7% | 20.0% | 18.4% |
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| Marriage & Family | | | |
| | Belleville, NJ | Westwood, NJ | United States |
| Married Population | 45.9% | 54.0% | 49.8% |
| Currently Married | 42.5% | 51.1% | 48.0% |
| Married but Separated | 3.4% | 2.9% | 1.8% |
| Single Population | 54.1% | 46.0% | 50.2% |
| Never Married | 37.6% | 28.2% | 33.8% |
| Divorced | 10.9% | 11.9% | 10.8% |
| Widowed | 5.5% | 5.9% | 5.6% |
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| Household Size | 2.79 | 2.57 | 2.60 |
| Households | 13,584 | 4,364 | 124,010,992 |
| Family Households | 9,249 | 3,051 | 80,755,759 |
| Married couple, w/children | 26.5% | 35.9% | 30.5% |
| Married couple, no children | 38.2% | 36.8% | 42.9% |
| Non Family Households | 4,335 | 1,313 | 43,255,233 |
| Single householder, w/children | 17.3% | 11.4% | 15.8% |
| Single householder, no children | 18.0% | 16.0% | 10.8% |