| | Belleville, NJ | Westfield, NJ | United States |
| Population | 37,914 | 30,950 | 329,725,481 |
| Female Population | 51.9% | 51.6% | 50.5% |
| Male Population | 48.1% | 48.4% | 49.5% |
| Median Age | 41.3 | 40.9 | 38.4 |
| space3 | | | |
| Population - 2010 | 35,822 | 30,336 | 309,327,143 |
| Population - 2000 | 35,928 | 29,644 | 285,036,114 |
| Population - 1990 | 34,213 | 28,870 | 251,960,433 |
| Pop. 2000 to Now | 1.4% | -1.4% | 15.6% |
| Pop. 1990 to Now | 6.5% | 1.3% | 30.8% |
| space4 | | | |
| Population Density | 11,351.5 | 4,602.9 | 93.4 |
| Land Area | 3.3 | 6.7 | 3,531,905.4 |
| Water Area | 0.1 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Belleville, NJ | Westfield, NJ | United States |
| Age 0 to 4 | 5.5% | 6.9% | 5.9% |
| Age 5 to 9 | 5.5% | 8.3% | 6.1% |
| Age 10 to 14 | 3.9% | 9.6% | 6.6% |
| Age 15 to 17 | 3.8% | 4.3% | 3.9% |
| Age 18 to 20 | 3.9% | 3.5% | 4.0% |
| Age 21 to 24 | 5.9% | 3.5% | 5.2% |
| Age 25 to 34 | 13.8% | 7.2% | 13.8% |
| Age 35 to 44 | 12.0% | 12.2% | 12.9% |
| Age 45 to 54 | 17.3% | 16.7% | 12.6% |
| Age 55 to 59 | 6.9% | 8.3% | 6.7% |
| Age 60 to 64 | 6.8% | 5.0% | 6.3% |
| Age 65 to 74 | 8.5% | 9.1% | 9.6% |
| Age 75 to 84 | 4.0% | 3.7% | 4.5% |
| Age 85 and over | 2.3% | 1.6% | 1.9% |
| space1 | | | |
| Race | | | |
| | Belleville, NJ | Westfield, NJ | United States |
| White | 30.3% | 77.6% | 59.4% |
| Black | 7.3% | 2.2% | 12.2% |
| Asian | 10.5% | 7.5% | 5.6% |
| Native American | 0.0% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 1.3% | 0.3% | 0.4% |
| Two or More Races | 2.0% | 3.3% | 3.2% |
| Hispanic | 48.7% | 9.0% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Belleville, NJ | Westfield, NJ | United States |
| Married Population | 45.9% | 65.1% | 49.8% |
| Currently Married | 42.5% | 64.4% | 48.0% |
| Married but Separated | 3.4% | 0.7% | 1.8% |
| Single Population | 54.1% | 34.9% | 50.2% |
| Never Married | 37.6% | 23.3% | 33.8% |
| Divorced | 10.9% | 6.7% | 10.8% |
| Widowed | 5.5% | 4.8% | 5.6% |
| space5 | | | |
| Household Size | 2.79 | 2.91 | 2.60 |
| Households | 13,584 | 10,514 | 124,010,992 |
| Family Households | 9,249 | 8,185 | 80,755,759 |
| Married couple, w/children | 26.5% | 47.0% | 30.5% |
| Married couple, no children | 38.2% | 43.6% | 42.9% |
| Non Family Households | 4,335 | 2,329 | 43,255,233 |
| Single householder, w/children | 17.3% | 6.5% | 15.8% |
| Single householder, no children | 18.0% | 3.0% | 10.8% |