| | Fairfield, AL | Round Rock, TX | United States |
| Population | 10,108 | 117,735 | 329,725,481 |
| Female Population | 56.4% | 50.8% | 50.5% |
| Male Population | 43.6% | 49.2% | 49.5% |
| Median Age | 32.1 | 35.6 | 38.4 |
| space3 | | | |
| Population - 2010 | 11,082 | 100,774 | 309,327,143 |
| Population - 2000 | 12,381 | 61,136 | 285,036,114 |
| Population - 1990 | 12,200 | 30,923 | 251,960,433 |
| Pop. 2000 to Now | -15.5% | 125.0% | 15.6% |
| Pop. 1990 to Now | -14.3% | 344.9% | 30.8% |
| space4 | | | |
| Population Density | 2,913.0 | 3,264.3 | 93.4 |
| Land Area | 3.5 | 36.1 | 3,531,905.4 |
| Water Area | 0.0 | 0.3 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Fairfield, AL | Round Rock, TX | United States |
| Age 0 to 4 | 5.9% | 5.9% | 5.9% |
| Age 5 to 9 | 6.9% | 7.4% | 6.1% |
| Age 10 to 14 | 5.6% | 7.8% | 6.6% |
| Age 15 to 17 | 3.1% | 4.9% | 3.9% |
| Age 18 to 20 | 10.1% | 3.5% | 4.0% |
| Age 21 to 24 | 4.8% | 5.6% | 5.2% |
| Age 25 to 34 | 15.6% | 13.9% | 13.8% |
| Age 35 to 44 | 9.3% | 16.9% | 12.9% |
| Age 45 to 54 | 10.0% | 14.2% | 12.6% |
| Age 55 to 59 | 4.3% | 5.6% | 6.7% |
| Age 60 to 64 | 9.2% | 4.7% | 6.3% |
| Age 65 to 74 | 11.9% | 6.5% | 9.6% |
| Age 75 to 84 | 2.1% | 1.9% | 4.5% |
| Age 85 and over | 1.3% | 1.1% | 1.9% |
| space1 | | | |
| Race | | | |
| | Fairfield, AL | Round Rock, TX | United States |
| White | 5.1% | 50.5% | 59.4% |
| Black | 92.1% | 10.5% | 12.2% |
| Asian | 0.0% | 6.9% | 5.6% |
| Native American | 0.0% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.2% | 0.2% |
| Other | 0.1% | 0.5% | 0.4% |
| Two or More Races | 1.7% | 3.5% | 3.2% |
| Hispanic | 1.0% | 27.7% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Fairfield, AL | Round Rock, TX | United States |
| Married Population | 26.6% | 52.7% | 49.8% |
| Currently Married | 24.1% | 51.2% | 48.0% |
| Married but Separated | 2.4% | 1.5% | 1.8% |
| Single Population | 73.4% | 47.3% | 50.2% |
| Never Married | 51.4% | 31.8% | 33.8% |
| Divorced | 16.4% | 11.5% | 10.8% |
| Widowed | 5.7% | 4.0% | 5.6% |
| space5 | | | |
| Household Size | 2.37 | 2.68 | 2.60 |
| Households | 3,944 | 43,705 | 124,010,992 |
| Family Households | 2,371 | 30,729 | 80,755,759 |
| Married couple, w/children | 9.7% | 42.0% | 30.5% |
| Married couple, no children | 29.3% | 32.0% | 42.9% |
| Non Family Households | 1,573 | 12,976 | 43,255,233 |
| Single householder, w/children | 43.1% | 15.7% | 15.8% |
| Single householder, no children | 17.9% | 10.3% | 10.8% |