| | Plymouth, MA | Portsmouth, NH | United States |
| Population | 60,987 | 21,897 | 329,725,481 |
| Female Population | 50.8% | 52.5% | 50.5% |
| Male Population | 49.2% | 47.6% | 49.5% |
| Median Age | 47.9 | 42.2 | 38.4 |
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| Population - 2010 | 56,654 | 20,770 | 309,327,143 |
| Population - 2000 | 51,701 | 20,784 | 285,036,114 |
| Population - 1990 | 45,608 | 25,925 | 251,960,433 |
| Pop. 2000 to Now | 21.2% | 3.7% | 15.6% |
| Pop. 1990 to Now | 37.4% | -16.9% | 30.8% |
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| Population Density | 632.3 | 1,398.4 | 93.4 |
| Land Area | 96.4 | 15.7 | 3,531,905.4 |
| Water Area | 37.5 | 1.2 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Plymouth, MA | Portsmouth, NH | United States |
| Age 0 to 4 | 4.2% | 3.5% | 5.9% |
| Age 5 to 9 | 5.0% | 4.9% | 6.1% |
| Age 10 to 14 | 4.7% | 3.9% | 6.6% |
| Age 15 to 17 | 3.1% | 2.9% | 3.9% |
| Age 18 to 20 | 2.3% | 1.8% | 4.0% |
| Age 21 to 24 | 3.8% | 4.9% | 5.2% |
| Age 25 to 34 | 12.3% | 17.5% | 13.8% |
| Age 35 to 44 | 11.0% | 13.6% | 12.9% |
| Age 45 to 54 | 13.2% | 13.0% | 12.6% |
| Age 55 to 59 | 8.6% | 6.5% | 6.7% |
| Age 60 to 64 | 8.7% | 7.3% | 6.3% |
| Age 65 to 74 | 14.6% | 12.0% | 9.6% |
| Age 75 to 84 | 6.2% | 5.9% | 4.5% |
| Age 85 and over | 2.3% | 2.4% | 1.9% |
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| Race | | | |
| | Plymouth, MA | Portsmouth, NH | United States |
| White | 91.2% | 87.3% | 59.4% |
| Black | 1.3% | 1.2% | 12.2% |
| Asian | 0.9% | 5.0% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.8% | 1.4% | 0.4% |
| Two or More Races | 3.0% | 2.9% | 3.2% |
| Hispanic | 2.7% | 2.3% | 18.4% |
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| Marriage & Family | | | |
| | Plymouth, MA | Portsmouth, NH | United States |
| Married Population | 57.5% | 48.6% | 49.8% |
| Currently Married | 56.2% | 48.0% | 48.0% |
| Married but Separated | 1.3% | 0.7% | 1.8% |
| Single Population | 42.5% | 51.4% | 50.2% |
| Never Married | 27.1% | 33.7% | 33.8% |
| Divorced | 9.4% | 12.3% | 10.8% |
| Widowed | 6.0% | 5.4% | 5.6% |
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| Household Size | 2.37 | 2.05 | 2.60 |
| Households | 24,985 | 10,462 | 124,010,992 |
| Family Households | 17,004 | 5,348 | 80,755,759 |
| Married couple, w/children | 25.8% | 23.9% | 30.5% |
| Married couple, no children | 56.0% | 55.7% | 42.9% |
| Non Family Households | 7,981 | 5,114 | 43,255,233 |
| Single householder, w/children | 9.7% | 10.9% | 15.8% |
| Single householder, no children | 8.4% | 9.6% | 10.8% |