| | Watertown Town, MA | Somerville, MA | United States |
| Population | 35,171 | 80,608 | 329,725,481 |
| Female Population | 53.6% | 49.9% | 50.5% |
| Male Population | 46.4% | 50.1% | 49.5% |
| Median Age | 39 | 31.8 | 38.4 |
| space3 | | | |
| Population - 2010 | 32,058 | 75,870 | 309,327,143 |
| Population - 2000 | 32,986 | 77,478 | 285,036,114 |
| Population - 1990 | 33,284 | 76,210 | 251,960,433 |
| Pop. 2000 to Now | 9.5% | 4.5% | 15.6% |
| Pop. 1990 to Now | 8.5% | 6.2% | 30.8% |
| space4 | | | |
| Population Density | 8,795.0 | 19,546.1 | 93.4 |
| Land Area | 4.0 | 4.1 | 3,531,905.4 |
| Water Area | 0.1 | 0.1 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Watertown Town, MA | Somerville, MA | United States |
| Age 0 to 4 | 5.8% | 4.0% | 5.9% |
| Age 5 to 9 | 2.9% | 3.0% | 6.1% |
| Age 10 to 14 | 3.5% | 2.2% | 6.6% |
| Age 15 to 17 | 2.2% | 1.3% | 3.9% |
| Age 18 to 20 | 2.6% | 4.7% | 4.0% |
| Age 21 to 24 | 3.6% | 9.6% | 5.2% |
| Age 25 to 34 | 21.4% | 33.1% | 13.8% |
| Age 35 to 44 | 15.5% | 14.5% | 12.9% |
| Age 45 to 54 | 11.2% | 9.2% | 12.6% |
| Age 55 to 59 | 6.5% | 4.6% | 6.7% |
| Age 60 to 64 | 7.4% | 4.3% | 6.3% |
| Age 65 to 74 | 9.9% | 5.5% | 9.6% |
| Age 75 to 84 | 5.0% | 2.7% | 4.5% |
| Age 85 and over | 2.5% | 1.1% | 1.9% |
| space1 | | | |
| Race | | | |
| | Watertown Town, MA | Somerville, MA | United States |
| White | 72.5% | 68.8% | 59.4% |
| Black | 3.6% | 4.8% | 12.2% |
| Asian | 11.9% | 9.3% | 5.6% |
| Native American | 0.2% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.2% | 0.9% | 0.4% |
| Two or More Races | 4.1% | 3.8% | 3.2% |
| Hispanic | 7.4% | 12.3% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Watertown Town, MA | Somerville, MA | United States |
| Married Population | 50.0% | 33.4% | 49.8% |
| Currently Married | 48.8% | 32.4% | 48.0% |
| Married but Separated | 1.1% | 1.0% | 1.8% |
| Single Population | 50.0% | 66.6% | 50.2% |
| Never Married | 37.7% | 58.0% | 33.8% |
| Divorced | 7.7% | 5.7% | 10.8% |
| Widowed | 4.7% | 2.9% | 5.6% |
| space5 | | | |
| Household Size | 2.18 | 2.25 | 2.60 |
| Households | 16,027 | 34,523 | 124,010,992 |
| Family Households | 8,748 | 13,639 | 80,755,759 |
| Married couple, w/children | 31.8% | 28.2% | 30.5% |
| Married couple, no children | 45.0% | 49.9% | 42.9% |
| Non Family Households | 7,279 | 20,884 | 43,255,233 |
| Single householder, w/children | 8.4% | 9.8% | 15.8% |
| Single householder, no children | 14.9% | 12.1% | 10.8% |