| | Decatur, AL | Round Rock, TX | United States |
| Population | 57,303 | 117,735 | 329,725,481 |
| Female Population | 50.7% | 50.8% | 50.5% |
| Male Population | 49.3% | 49.2% | 49.5% |
| Median Age | 40.6 | 35.6 | 38.4 |
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| Population - 2010 | 55,810 | 100,774 | 309,327,143 |
| Population - 2000 | 53,929 | 61,136 | 285,036,114 |
| Population - 1990 | 48,761 | 30,923 | 251,960,433 |
| Pop. 2000 to Now | 0.8% | 125.0% | 15.6% |
| Pop. 1990 to Now | 11.5% | 344.9% | 30.8% |
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| Population Density | 1,058.9 | 3,264.3 | 93.4 |
| Land Area | 54.1 | 36.1 | 3,531,905.4 |
| Water Area | 6.6 | 0.3 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Decatur, AL | Round Rock, TX | United States |
| Age 0 to 4 | 6.5% | 5.9% | 5.9% |
| Age 5 to 9 | 6.1% | 7.4% | 6.1% |
| Age 10 to 14 | 6.1% | 7.8% | 6.6% |
| Age 15 to 17 | 4.4% | 4.9% | 3.9% |
| Age 18 to 20 | 2.6% | 3.5% | 4.0% |
| Age 21 to 24 | 5.0% | 5.6% | 5.2% |
| Age 25 to 34 | 12.1% | 13.9% | 13.8% |
| Age 35 to 44 | 12.4% | 16.9% | 12.9% |
| Age 45 to 54 | 12.4% | 14.2% | 12.6% |
| Age 55 to 59 | 6.2% | 5.6% | 6.7% |
| Age 60 to 64 | 7.2% | 4.7% | 6.3% |
| Age 65 to 74 | 10.9% | 6.5% | 9.6% |
| Age 75 to 84 | 5.9% | 1.9% | 4.5% |
| Age 85 and over | 2.2% | 1.1% | 1.9% |
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| Race | | | |
| | Decatur, AL | Round Rock, TX | United States |
| White | 59.9% | 50.5% | 59.4% |
| Black | 22.6% | 10.5% | 12.2% |
| Asian | 0.4% | 6.9% | 5.6% |
| Native American | 0.1% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.2% | 0.2% |
| Other | 0.4% | 0.5% | 0.4% |
| Two or More Races | 2.5% | 3.5% | 3.2% |
| Hispanic | 14.1% | 27.7% | 18.4% |
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| Marriage & Family | | | |
| | Decatur, AL | Round Rock, TX | United States |
| Married Population | 49.9% | 52.7% | 49.8% |
| Currently Married | 47.2% | 51.2% | 48.0% |
| Married but Separated | 2.6% | 1.5% | 1.8% |
| Single Population | 50.1% | 47.3% | 50.2% |
| Never Married | 28.3% | 31.8% | 33.8% |
| Divorced | 13.3% | 11.5% | 10.8% |
| Widowed | 8.5% | 4.0% | 5.6% |
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| Household Size | 2.37 | 2.68 | 2.60 |
| Households | 23,797 | 43,705 | 124,010,992 |
| Family Households | 14,199 | 30,729 | 80,755,759 |
| Married couple, w/children | 25.6% | 42.0% | 30.5% |
| Married couple, no children | 46.0% | 32.0% | 42.9% |
| Non Family Households | 9,598 | 12,976 | 43,255,233 |
| Single householder, w/children | 17.9% | 15.7% | 15.8% |
| Single householder, no children | 10.6% | 10.3% | 10.8% |