| | Westfield, MA | Chicago, IL | United States |
| Population | 40,922 | 2,742,119 | 329,725,481 |
| Female Population | 52.0% | 51.2% | 50.5% |
| Male Population | 48.0% | 48.8% | 49.5% |
| Median Age | 38.5 | 35.1 | 38.4 |
| space3 | | | |
| Population - 2010 | 41,121 | 2,697,480 | 309,327,143 |
| Population - 2000 | 40,072 | 2,896,016 | 285,036,114 |
| Population - 1990 | 38,372 | 2,783,726 | 251,960,433 |
| Pop. 2000 to Now | 2.5% | -7.5% | 15.6% |
| Pop. 1990 to Now | 7.0% | -3.8% | 30.8% |
| space4 | | | |
| Population Density | 884.7 | 12,060.3 | 93.4 |
| Land Area | 46.3 | 227.4 | 3,531,905.4 |
| Water Area | 1.1 | 6.8 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Westfield, MA | Chicago, IL | United States |
| Age 0 to 4 | 4.1% | 5.9% | 5.9% |
| Age 5 to 9 | 5.8% | 5.4% | 6.1% |
| Age 10 to 14 | 5.1% | 5.8% | 6.6% |
| Age 15 to 17 | 4.0% | 3.3% | 3.9% |
| Age 18 to 20 | 8.0% | 3.6% | 4.0% |
| Age 21 to 24 | 5.9% | 6.0% | 5.2% |
| Age 25 to 34 | 13.7% | 19.8% | 13.8% |
| Age 35 to 44 | 11.2% | 14.3% | 12.9% |
| Age 45 to 54 | 11.3% | 11.9% | 12.6% |
| Age 55 to 59 | 7.0% | 5.8% | 6.7% |
| Age 60 to 64 | 6.4% | 5.4% | 6.3% |
| Age 65 to 74 | 10.7% | 7.6% | 9.6% |
| Age 75 to 84 | 4.0% | 3.6% | 4.5% |
| Age 85 and over | 2.6% | 1.6% | 1.9% |
| space1 | | | |
| Race | | | |
| | Westfield, MA | Chicago, IL | United States |
| White | 83.0% | 33.1% | 59.4% |
| Black | 1.5% | 28.8% | 12.2% |
| Asian | 4.0% | 6.8% | 5.6% |
| Native American | 0.0% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.1% | 0.3% | 0.4% |
| Two or More Races | 1.9% | 2.2% | 3.2% |
| Hispanic | 9.5% | 28.7% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Westfield, MA | Chicago, IL | United States |
| Married Population | 46.5% | 37.8% | 49.8% |
| Currently Married | 45.2% | 35.5% | 48.0% |
| Married but Separated | 1.3% | 2.3% | 1.8% |
| Single Population | 53.5% | 62.2% | 50.2% |
| Never Married | 37.9% | 48.9% | 33.8% |
| Divorced | 10.0% | 8.3% | 10.8% |
| Widowed | 5.6% | 5.0% | 5.6% |
| space5 | | | |
| Household Size | 2.47 | 2.41 | 2.60 |
| Households | 15,292 | 1,112,581 | 124,010,992 |
| Family Households | 10,083 | 574,825 | 80,755,759 |
| Married couple, w/children | 27.2% | 27.1% | 30.5% |
| Married couple, no children | 46.4% | 34.8% | 42.9% |
| Non Family Households | 5,209 | 537,756 | 43,255,233 |
| Single householder, w/children | 14.4% | 21.0% | 15.8% |
| Single householder, no children | 12.0% | 17.1% | 10.8% |