| | Fairfield, IA | Brookfield, WI | United States |
| Population | 9,653 | 41,020 | 329,725,481 |
| Female Population | 49.5% | 50.4% | 50.5% |
| Male Population | 50.5% | 49.6% | 49.5% |
| Median Age | 39.1 | 44.1 | 38.4 |
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| Population - 2010 | 9,479 | 37,915 | 309,327,143 |
| Population - 2000 | 9,509 | 38,649 | 285,036,114 |
| Population - 1990 | 9,768 | 35,184 | 251,960,433 |
| Pop. 2000 to Now | 10.4% | 1.3% | 15.6% |
| Pop. 1990 to Now | 7.5% | 11.3% | 30.8% |
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| Population Density | 1,475.3 | 1,503.1 | 93.4 |
| Land Area | 6.5 | 27.3 | 3,531,905.4 |
| Water Area | 0.2 | 0.4 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Fairfield, IA | Brookfield, WI | United States |
| Age 0 to 4 | 5.1% | 5.7% | 5.9% |
| Age 5 to 9 | 7.2% | 6.4% | 6.1% |
| Age 10 to 14 | 5.0% | 7.2% | 6.6% |
| Age 15 to 17 | 4.0% | 4.6% | 3.9% |
| Age 18 to 20 | 2.3% | 2.9% | 4.0% |
| Age 21 to 24 | 5.9% | 2.9% | 5.2% |
| Age 25 to 34 | 16.8% | 8.5% | 13.8% |
| Age 35 to 44 | 10.5% | 13.1% | 12.9% |
| Age 45 to 54 | 10.5% | 12.4% | 12.6% |
| Age 55 to 59 | 5.0% | 7.5% | 6.7% |
| Age 60 to 64 | 5.0% | 6.7% | 6.3% |
| Age 65 to 74 | 14.8% | 12.7% | 9.6% |
| Age 75 to 84 | 5.0% | 6.1% | 4.5% |
| Age 85 and over | 3.0% | 3.3% | 1.9% |
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| Race | | | |
| | Fairfield, IA | Brookfield, WI | United States |
| White | 72.6% | 80.9% | 59.4% |
| Black | 10.4% | 0.9% | 12.2% |
| Asian | 8.5% | 11.4% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.4% | 0.2% | 0.4% |
| Two or More Races | 3.8% | 3.4% | 3.2% |
| Hispanic | 4.3% | 3.1% | 18.4% |
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| Marriage & Family | | | |
| | Fairfield, IA | Brookfield, WI | United States |
| Married Population | 39.5% | 65.3% | 49.8% |
| Currently Married | 38.4% | 64.9% | 48.0% |
| Married but Separated | 1.2% | 0.4% | 1.8% |
| Single Population | 60.5% | 34.7% | 50.2% |
| Never Married | 36.1% | 21.1% | 33.8% |
| Divorced | 19.6% | 7.4% | 10.8% |
| Widowed | 4.7% | 6.2% | 5.6% |
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| Household Size | 2.21 | 2.55 | 2.60 |
| Households | 4,045 | 15,929 | 124,010,992 |
| Family Households | 2,081 | 11,595 | 80,755,759 |
| Married couple, w/children | 25.3% | 38.0% | 30.5% |
| Married couple, no children | 46.0% | 53.1% | 42.9% |
| Non Family Households | 1,964 | 4,334 | 43,255,233 |
| Single householder, w/children | 20.7% | 4.8% | 15.8% |
| Single householder, no children | 8.0% | 4.1% | 10.8% |