| | Greenbrier, TN | Portland, TN | United States |
| Population | 6,845 | 13,031 | 329,725,481 |
| Female Population | 51.0% | 49.9% | 50.5% |
| Male Population | 49.0% | 50.2% | 49.5% |
| Median Age | 35.3 | 33.3 | 38.4 |
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| Population - 2010 | 6,481 | 11,530 | 309,327,143 |
| Population - 2000 | 4,940 | 8,458 | 285,036,114 |
| Population - 1990 | 2,873 | 5,165 | 251,960,433 |
| Pop. 2000 to Now | 39.8% | 57.3% | 15.6% |
| Pop. 1990 to Now | 140.3% | 157.5% | 30.8% |
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| Population Density | 968.5 | 977.1 | 93.4 |
| Land Area | 7.1 | 13.3 | 3,531,905.4 |
| Water Area | 0.0 | 0.0 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Greenbrier, TN | Portland, TN | United States |
| Age 0 to 4 | 7.3% | 6.8% | 5.9% |
| Age 5 to 9 | 7.8% | 7.5% | 6.1% |
| Age 10 to 14 | 11.8% | 6.5% | 6.6% |
| Age 15 to 17 | 5.6% | 6.5% | 3.9% |
| Age 18 to 20 | 1.6% | 5.4% | 4.0% |
| Age 21 to 24 | 2.4% | 5.6% | 5.2% |
| Age 25 to 34 | 12.9% | 14.2% | 13.8% |
| Age 35 to 44 | 16.3% | 14.0% | 12.9% |
| Age 45 to 54 | 11.7% | 12.9% | 12.6% |
| Age 55 to 59 | 7.8% | 4.9% | 6.7% |
| Age 60 to 64 | 5.3% | 5.8% | 6.3% |
| Age 65 to 74 | 5.9% | 7.2% | 9.6% |
| Age 75 to 84 | 2.6% | 1.9% | 4.5% |
| Age 85 and over | 0.8% | 0.8% | 1.9% |
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| Race | | | |
| | Greenbrier, TN | Portland, TN | United States |
| White | 89.9% | 86.9% | 59.4% |
| Black | 1.3% | 2.1% | 12.2% |
| Asian | 2.1% | 0.2% | 5.6% |
| Native American | 0.1% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.1% | 0.4% |
| Two or More Races | 2.0% | 1.3% | 3.2% |
| Hispanic | 4.6% | 9.1% | 18.4% |
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| Marriage & Family | | | |
| | Greenbrier, TN | Portland, TN | United States |
| Married Population | 53.7% | 49.5% | 49.8% |
| Currently Married | 52.7% | 46.4% | 48.0% |
| Married but Separated | 1.1% | 3.2% | 1.8% |
| Single Population | 46.3% | 50.5% | 50.2% |
| Never Married | 25.6% | 31.0% | 33.8% |
| Divorced | 14.7% | 15.1% | 10.8% |
| Widowed | 6.0% | 4.4% | 5.6% |
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| Household Size | 3.31 | 2.77 | 2.60 |
| Households | 2,070 | 4,688 | 124,010,992 |
| Family Households | 1,619 | 3,330 | 80,755,759 |
| Married couple, w/children | 37.5% | 38.2% | 30.5% |
| Married couple, no children | 37.1% | 30.9% | 42.9% |
| Non Family Households | 451 | 1,358 | 43,255,233 |
| Single householder, w/children | 19.3% | 22.8% | 15.8% |
| Single householder, no children | 6.1% | 8.1% | 10.8% |