| | Fairfield, ID | Twin Falls, ID | United States |
| Population | 493 | 51,223 | 329,725,481 |
| Female Population | 66.7% | 50.6% | 50.5% |
| Male Population | 33.3% | 49.5% | 49.5% |
| Median Age | 35.5 | 34 | 38.4 |
| space3 | | | |
| Population - 2010 | 415 | 44,511 | 309,327,143 |
| Population - 2000 | 395 | 34,469 | 285,036,114 |
| Population - 1990 | 371 | 27,591 | 251,960,433 |
| Pop. 2000 to Now | 2.0% | 49.2% | 15.6% |
| Pop. 1990 to Now | 8.6% | 86.3% | 30.8% |
| space4 | | | |
| Population Density | 562.8 | 2,650.6 | 93.4 |
| Land Area | 0.9 | 19.3 | 3,531,905.4 |
| Water Area | 0.0 | 0.1 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Fairfield, ID | Twin Falls, ID | United States |
| Age 0 to 4 | 4.5% | 6.9% | 5.9% |
| Age 5 to 9 | 15.0% | 8.2% | 6.1% |
| Age 10 to 14 | 8.5% | 7.0% | 6.6% |
| Age 15 to 17 | 3.0% | 3.4% | 3.9% |
| Age 18 to 20 | 4.3% | 3.6% | 4.0% |
| Age 21 to 24 | 0.0% | 6.2% | 5.2% |
| Age 25 to 34 | 13.6% | 15.7% | 13.8% |
| Age 35 to 44 | 29.0% | 13.7% | 12.9% |
| Age 45 to 54 | 5.1% | 10.2% | 12.6% |
| Age 55 to 59 | 4.7% | 4.9% | 6.7% |
| Age 60 to 64 | 2.8% | 4.9% | 6.3% |
| Age 65 to 74 | 2.4% | 9.0% | 9.6% |
| Age 75 to 84 | 6.3% | 5.0% | 4.5% |
| Age 85 and over | 0.8% | 1.2% | 1.9% |
| space1 | | | |
| Race | | | |
| | Fairfield, ID | Twin Falls, ID | United States |
| White | 93.3% | 77.3% | 59.4% |
| Black | 0.0% | 1.1% | 12.2% |
| Asian | 0.0% | 2.5% | 5.6% |
| Native American | 0.0% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.4% | 0.4% |
| Two or More Races | 2.6% | 4.3% | 3.2% |
| Hispanic | 4.1% | 14.3% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Fairfield, ID | Twin Falls, ID | United States |
| Married Population | 28.2% | 53.4% | 49.8% |
| Currently Married | 28.2% | 51.6% | 48.0% |
| Married but Separated | 0.0% | 1.7% | 1.8% |
| Single Population | 71.8% | 46.6% | 50.2% |
| Never Married | 26.5% | 26.8% | 33.8% |
| Divorced | 41.4% | 13.7% | 10.8% |
| Widowed | 3.9% | 6.1% | 5.6% |
| space5 | | | |
| Household Size | 4.07 | 2.58 | 2.60 |
| Households | 121 | 19,538 | 124,010,992 |
| Family Households | 54 | 12,555 | 80,755,759 |
| Married couple, w/children | 51.9% | 35.3% | 30.5% |
| Married couple, no children | 37.0% | 41.6% | 42.9% |
| Non Family Households | 67 | 6,983 | 43,255,233 |
| Single householder, w/children | 11.1% | 16.4% | 15.8% |
| Single householder, no children | 0.0% | 6.8% | 10.8% |