| | Fairfield, IL | Mobile, AL | United States |
| Population | 4,982 | 187,445 | 329,725,481 |
| Female Population | 50.2% | 52.8% | 50.5% |
| Male Population | 49.8% | 47.2% | 49.5% |
| Median Age | 42.6 | 37.8 | 38.4 |
| space3 | | | |
| Population - 2010 | 5,160 | 194,590 | 309,327,143 |
| Population - 2000 | 5,421 | 198,915 | 285,036,114 |
| Population - 1990 | 5,439 | 196,278 | 251,960,433 |
| Pop. 2000 to Now | -9.8% | -5.6% | 15.6% |
| Pop. 1990 to Now | -10.1% | -4.4% | 30.8% |
| space4 | | | |
| Population Density | 1,237.2 | 1,344.7 | 93.4 |
| Land Area | 4.0 | 139.4 | 3,531,905.4 |
| Water Area | 0.0 | 40.7 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Fairfield, IL | Mobile, AL | United States |
| Age 0 to 4 | 6.9% | 5.8% | 5.9% |
| Age 5 to 9 | 5.3% | 5.7% | 6.1% |
| Age 10 to 14 | 5.7% | 6.4% | 6.6% |
| Age 15 to 17 | 3.3% | 3.7% | 3.9% |
| Age 18 to 20 | 4.6% | 4.3% | 4.0% |
| Age 21 to 24 | 4.8% | 5.9% | 5.2% |
| Age 25 to 34 | 11.9% | 14.8% | 13.8% |
| Age 35 to 44 | 11.8% | 12.0% | 12.9% |
| Age 45 to 54 | 11.0% | 11.6% | 12.6% |
| Age 55 to 59 | 8.7% | 6.5% | 6.7% |
| Age 60 to 64 | 5.5% | 6.7% | 6.3% |
| Age 65 to 74 | 9.8% | 9.9% | 9.6% |
| Age 75 to 84 | 6.5% | 4.7% | 4.5% |
| Age 85 and over | 4.3% | 2.0% | 1.9% |
| space1 | | | |
| Race | | | |
| | Fairfield, IL | Mobile, AL | United States |
| White | 93.8% | 40.5% | 59.4% |
| Black | 3.1% | 52.4% | 12.2% |
| Asian | 0.6% | 1.8% | 5.6% |
| Native American | 0.0% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.3% | 0.4% |
| Two or More Races | 1.0% | 2.2% | 3.2% |
| Hispanic | 1.6% | 2.6% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Fairfield, IL | Mobile, AL | United States |
| Married Population | 47.6% | 39.4% | 49.8% |
| Currently Married | 47.3% | 36.7% | 48.0% |
| Married but Separated | 0.3% | 2.7% | 1.8% |
| Single Population | 52.4% | 60.6% | 50.2% |
| Never Married | 24.2% | 40.7% | 33.8% |
| Divorced | 17.1% | 12.7% | 10.8% |
| Widowed | 11.1% | 7.1% | 5.6% |
| space5 | | | |
| Household Size | 2.12 | 2.35 | 2.60 |
| Households | 2,333 | 77,515 | 124,010,992 |
| Family Households | 1,385 | 42,610 | 80,755,759 |
| Married couple, w/children | 30.7% | 21.9% | 30.5% |
| Married couple, no children | 40.1% | 38.9% | 42.9% |
| Non Family Households | 948 | 34,905 | 43,255,233 |
| Single householder, w/children | 21.5% | 23.4% | 15.8% |
| Single householder, no children | 7.7% | 15.8% | 10.8% |