| | Gardner, MA | San Diego, CA | United States |
| Population | 21,183 | 1,385,398 | 329,725,481 |
| Female Population | 47.0% | 49.0% | 50.5% |
| Male Population | 53.0% | 51.0% | 49.5% |
| Median Age | 44.3 | 35.4 | 38.4 |
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| Population - 2010 | 20,174 | 1,305,632 | 309,327,143 |
| Population - 2000 | 20,770 | 1,223,400 | 285,036,114 |
| Population - 1990 | 20,125 | 1,110,549 | 251,960,433 |
| Pop. 2000 to Now | -0.9% | 16.3% | 15.6% |
| Pop. 1990 to Now | 2.3% | 28.1% | 30.8% |
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| Population Density | 959.9 | 4,255.0 | 93.4 |
| Land Area | 22.1 | 325.6 | 3,531,905.4 |
| Water Area | 0.9 | 46.8 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Gardner, MA | San Diego, CA | United States |
| Age 0 to 4 | 5.3% | 5.6% | 5.9% |
| Age 5 to 9 | 4.6% | 5.4% | 6.1% |
| Age 10 to 14 | 5.4% | 5.5% | 6.6% |
| Age 15 to 17 | 2.3% | 3.1% | 3.9% |
| Age 18 to 20 | 2.6% | 4.4% | 4.0% |
| Age 21 to 24 | 3.2% | 6.3% | 5.2% |
| Age 25 to 34 | 15.8% | 19.0% | 13.8% |
| Age 35 to 44 | 11.7% | 14.3% | 12.9% |
| Age 45 to 54 | 13.0% | 12.2% | 12.6% |
| Age 55 to 59 | 7.7% | 5.7% | 6.7% |
| Age 60 to 64 | 10.3% | 5.2% | 6.3% |
| Age 65 to 74 | 10.2% | 8.0% | 9.6% |
| Age 75 to 84 | 4.9% | 3.7% | 4.5% |
| Age 85 and over | 3.1% | 1.6% | 1.9% |
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| Race | | | |
| | Gardner, MA | San Diego, CA | United States |
| White | 82.5% | 42.0% | 59.4% |
| Black | 2.0% | 5.7% | 12.2% |
| Asian | 3.1% | 16.9% | 5.6% |
| Native American | 0.0% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.4% | 0.2% |
| Other | 0.3% | 0.4% | 0.4% |
| Two or More Races | 2.9% | 4.3% | 3.2% |
| Hispanic | 9.1% | 30.1% | 18.4% |
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| Marriage & Family | | | |
| | Gardner, MA | San Diego, CA | United States |
| Married Population | 46.6% | 46.4% | 49.8% |
| Currently Married | 44.7% | 44.8% | 48.0% |
| Married but Separated | 1.9% | 1.6% | 1.8% |
| Single Population | 53.4% | 53.6% | 50.2% |
| Never Married | 33.8% | 40.1% | 33.8% |
| Divorced | 12.4% | 9.5% | 10.8% |
| Widowed | 7.2% | 4.0% | 5.6% |
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| Household Size | 2.25 | 2.64 | 2.60 |
| Households | 8,841 | 509,347 | 124,010,992 |
| Family Households | 5,446 | 304,499 | 80,755,759 |
| Married couple, w/children | 21.3% | 33.2% | 30.5% |
| Married couple, no children | 48.6% | 41.9% | 42.9% |
| Non Family Households | 3,395 | 204,848 | 43,255,233 |
| Single householder, w/children | 19.8% | 13.4% | 15.8% |
| Single householder, no children | 10.3% | 11.6% | 10.8% |