| | Fort Atkinson, WI | Eureka, MT | United States |
| Population | 12,529 | 1,447 | 329,725,481 |
| Female Population | 49.7% | 52.7% | 50.5% |
| Male Population | 50.3% | 47.3% | 49.5% |
| Median Age | 41.6 | 42.4 | 38.4 |
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| Population - 2010 | 12,393 | 1,403 | 309,327,143 |
| Population - 2000 | 11,621 | 1,017 | 285,036,114 |
| Population - 1990 | 10,227 | 1,043 | 251,960,433 |
| Pop. 2000 to Now | 6.5% | 42.1% | 15.6% |
| Pop. 1990 to Now | 21.0% | 38.5% | 30.8% |
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| Population Density | 2,214.0 | 1,400.8 | 93.4 |
| Land Area | 5.7 | 1.0 | 3,531,905.4 |
| Water Area | 0.1 | 0.0 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Fort Atkinson, WI | Eureka, MT | United States |
| Age 0 to 4 | 4.4% | 4.8% | 5.9% |
| Age 5 to 9 | 4.5% | 4.4% | 6.1% |
| Age 10 to 14 | 7.5% | 7.0% | 6.6% |
| Age 15 to 17 | 4.8% | 7.3% | 3.9% |
| Age 18 to 20 | 2.9% | 2.6% | 4.0% |
| Age 21 to 24 | 4.4% | 1.0% | 5.2% |
| Age 25 to 34 | 13.7% | 14.9% | 13.8% |
| Age 35 to 44 | 12.1% | 11.7% | 12.9% |
| Age 45 to 54 | 11.9% | 12.3% | 12.6% |
| Age 55 to 59 | 7.9% | 9.4% | 6.7% |
| Age 60 to 64 | 8.2% | 7.4% | 6.3% |
| Age 65 to 74 | 10.7% | 12.2% | 9.6% |
| Age 75 to 84 | 4.7% | 2.1% | 4.5% |
| Age 85 and over | 2.6% | 3.0% | 1.9% |
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| Race | | | |
| | Fort Atkinson, WI | Eureka, MT | United States |
| White | 87.2% | 89.1% | 59.4% |
| Black | 1.0% | 0.0% | 12.2% |
| Asian | 0.3% | 0.0% | 5.6% |
| Native American | 0.1% | 3.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.1% | 0.0% | 0.2% |
| Other | 0.0% | 0.0% | 0.4% |
| Two or More Races | 3.2% | 1.2% | 3.2% |
| Hispanic | 8.1% | 6.6% | 18.4% |
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| Marriage & Family | | | |
| | Fort Atkinson, WI | Eureka, MT | United States |
| Married Population | 51.8% | 44.9% | 49.8% |
| Currently Married | 51.3% | 43.9% | 48.0% |
| Married but Separated | 0.5% | 1.0% | 1.8% |
| Single Population | 48.2% | 55.1% | 50.2% |
| Never Married | 29.2% | 34.4% | 33.8% |
| Divorced | 13.0% | 14.8% | 10.8% |
| Widowed | 6.1% | 5.9% | 5.6% |
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| Household Size | 2.32 | 2.46 | 2.60 |
| Households | 5,305 | 575 | 124,010,992 |
| Family Households | 3,300 | 279 | 80,755,759 |
| Married couple, w/children | 27.8% | 26.2% | 30.5% |
| Married couple, no children | 51.2% | 58.1% | 42.9% |
| Non Family Households | 2,005 | 296 | 43,255,233 |
| Single householder, w/children | 13.8% | 11.5% | 15.8% |
| Single householder, no children | 7.2% | 4.3% | 10.8% |