| | Gardner, MA | Watertown Town, MA | United States |
| Population | 21,183 | 35,171 | 329,725,481 |
| Female Population | 47.0% | 53.6% | 50.5% |
| Male Population | 53.0% | 46.4% | 49.5% |
| Median Age | 44.3 | 39 | 38.4 |
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| Population - 2010 | 20,174 | 32,058 | 309,327,143 |
| Population - 2000 | 20,770 | 32,986 | 285,036,114 |
| Population - 1990 | 20,125 | 33,284 | 251,960,433 |
| Pop. 2000 to Now | -0.9% | 9.5% | 15.6% |
| Pop. 1990 to Now | 2.3% | 8.5% | 30.8% |
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| Population Density | 959.9 | 8,795.0 | 93.4 |
| Land Area | 22.1 | 4.0 | 3,531,905.4 |
| Water Area | 0.9 | 0.1 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Gardner, MA | Watertown Town, MA | United States |
| Age 0 to 4 | 5.3% | 5.8% | 5.9% |
| Age 5 to 9 | 4.6% | 2.9% | 6.1% |
| Age 10 to 14 | 5.4% | 3.5% | 6.6% |
| Age 15 to 17 | 2.3% | 2.2% | 3.9% |
| Age 18 to 20 | 2.6% | 2.6% | 4.0% |
| Age 21 to 24 | 3.2% | 3.6% | 5.2% |
| Age 25 to 34 | 15.8% | 21.4% | 13.8% |
| Age 35 to 44 | 11.7% | 15.5% | 12.9% |
| Age 45 to 54 | 13.0% | 11.2% | 12.6% |
| Age 55 to 59 | 7.7% | 6.5% | 6.7% |
| Age 60 to 64 | 10.3% | 7.4% | 6.3% |
| Age 65 to 74 | 10.2% | 9.9% | 9.6% |
| Age 75 to 84 | 4.9% | 5.0% | 4.5% |
| Age 85 and over | 3.1% | 2.5% | 1.9% |
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| Race | | | |
| | Gardner, MA | Watertown Town, MA | United States |
| White | 82.5% | 72.5% | 59.4% |
| Black | 2.0% | 3.6% | 12.2% |
| Asian | 3.1% | 11.9% | 5.6% |
| Native American | 0.0% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.3% | 0.2% | 0.4% |
| Two or More Races | 2.9% | 4.1% | 3.2% |
| Hispanic | 9.1% | 7.4% | 18.4% |
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| Marriage & Family | | | |
| | Gardner, MA | Watertown Town, MA | United States |
| Married Population | 46.6% | 50.0% | 49.8% |
| Currently Married | 44.7% | 48.8% | 48.0% |
| Married but Separated | 1.9% | 1.1% | 1.8% |
| Single Population | 53.4% | 50.0% | 50.2% |
| Never Married | 33.8% | 37.7% | 33.8% |
| Divorced | 12.4% | 7.7% | 10.8% |
| Widowed | 7.2% | 4.7% | 5.6% |
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| Household Size | 2.25 | 2.18 | 2.60 |
| Households | 8,841 | 16,027 | 124,010,992 |
| Family Households | 5,446 | 8,748 | 80,755,759 |
| Married couple, w/children | 21.3% | 31.8% | 30.5% |
| Married couple, no children | 48.6% | 45.0% | 42.9% |
| Non Family Households | 3,395 | 7,279 | 43,255,233 |
| Single householder, w/children | 19.8% | 8.4% | 15.8% |
| Single householder, no children | 10.3% | 14.9% | 10.8% |