| | Merrimack, NH | South Windsor, CT | United States |
| Population | 26,762 | 26,719 | 329,725,481 |
| Female Population | 50.0% | 49.3% | 50.5% |
| Male Population | 50.0% | 50.7% | 49.5% |
| Median Age | 42.1 | 41.4 | 38.4 |
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| Population - 2010 | 25,507 | 25,705 | 309,327,143 |
| Population - 2000 | 25,119 | 24,412 | 285,036,114 |
| Population - 1990 | 22,156 | 22,090 | 251,960,433 |
| Pop. 2000 to Now | 9.3% | 6.9% | 15.6% |
| Pop. 1990 to Now | 23.9% | 18.2% | 30.8% |
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| Population Density | 822.7 | 952.1 | 93.4 |
| Land Area | 32.5 | 28.1 | 3,531,905.4 |
| Water Area | 0.9 | 0.6 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Merrimack, NH | South Windsor, CT | United States |
| Age 0 to 4 | 5.2% | 6.2% | 5.9% |
| Age 5 to 9 | 4.9% | 6.7% | 6.1% |
| Age 10 to 14 | 8.3% | 7.1% | 6.6% |
| Age 15 to 17 | 4.4% | 3.5% | 3.9% |
| Age 18 to 20 | 3.5% | 3.4% | 4.0% |
| Age 21 to 24 | 3.2% | 4.2% | 5.2% |
| Age 25 to 34 | 12.7% | 9.0% | 13.8% |
| Age 35 to 44 | 12.0% | 15.1% | 12.9% |
| Age 45 to 54 | 13.8% | 14.8% | 12.6% |
| Age 55 to 59 | 8.3% | 6.5% | 6.7% |
| Age 60 to 64 | 6.8% | 7.7% | 6.3% |
| Age 65 to 74 | 11.2% | 9.3% | 9.6% |
| Age 75 to 84 | 4.7% | 4.2% | 4.5% |
| Age 85 and over | 1.0% | 2.4% | 1.9% |
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| Race | | | |
| | Merrimack, NH | South Windsor, CT | United States |
| White | 86.4% | 68.0% | 59.4% |
| Black | 1.1% | 3.9% | 12.2% |
| Asian | 4.4% | 17.2% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.4% | 0.3% | 0.4% |
| Two or More Races | 2.7% | 4.0% | 3.2% |
| Hispanic | 5.0% | 6.6% | 18.4% |
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| Marriage & Family | | | |
| | Merrimack, NH | South Windsor, CT | United States |
| Married Population | 58.0% | 64.5% | 49.8% |
| Currently Married | 57.4% | 63.7% | 48.0% |
| Married but Separated | 0.6% | 0.8% | 1.8% |
| Single Population | 42.0% | 35.5% | 50.2% |
| Never Married | 27.5% | 24.6% | 33.8% |
| Divorced | 10.7% | 6.5% | 10.8% |
| Widowed | 3.8% | 4.4% | 5.6% |
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| Household Size | 2.66 | 2.68 | 2.60 |
| Households | 10,009 | 9,926 | 124,010,992 |
| Family Households | 7,271 | 7,516 | 80,755,759 |
| Married couple, w/children | 31.9% | 42.8% | 30.5% |
| Married couple, no children | 52.7% | 44.3% | 42.9% |
| Non Family Households | 2,738 | 2,410 | 43,255,233 |
| Single householder, w/children | 9.0% | 9.1% | 15.8% |
| Single householder, no children | 6.4% | 3.8% | 10.8% |