| | Janesville, WI | Butler, PA | United States |
| Population | 65,506 | 13,452 | 329,725,481 |
| Female Population | 51.1% | 48.6% | 50.5% |
| Male Population | 48.9% | 51.4% | 49.5% |
| Median Age | 39.7 | 39.4 | 38.4 |
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| Population - 2010 | 63,632 | 13,756 | 309,327,143 |
| Population - 2000 | 59,498 | 15,121 | 285,036,114 |
| Population - 1990 | 52,133 | 15,714 | 251,960,433 |
| Pop. 2000 to Now | 8.7% | -15.0% | 15.6% |
| Pop. 1990 to Now | 24.0% | -18.2% | 30.8% |
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| Population Density | 1,924.1 | 4,947.4 | 93.4 |
| Land Area | 34.0 | 2.7 | 3,531,905.4 |
| Water Area | 0.6 | 0.0 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Janesville, WI | Butler, PA | United States |
| Age 0 to 4 | 5.1% | 5.4% | 5.9% |
| Age 5 to 9 | 6.7% | 4.5% | 6.1% |
| Age 10 to 14 | 6.9% | 6.5% | 6.6% |
| Age 15 to 17 | 4.1% | 3.8% | 3.9% |
| Age 18 to 20 | 2.9% | 4.8% | 4.0% |
| Age 21 to 24 | 5.0% | 5.9% | 5.2% |
| Age 25 to 34 | 12.7% | 13.1% | 13.8% |
| Age 35 to 44 | 13.7% | 12.2% | 12.9% |
| Age 45 to 54 | 12.6% | 14.5% | 12.6% |
| Age 55 to 59 | 6.8% | 8.5% | 6.7% |
| Age 60 to 64 | 6.9% | 7.8% | 6.3% |
| Age 65 to 74 | 9.2% | 8.6% | 9.6% |
| Age 75 to 84 | 5.0% | 3.0% | 4.5% |
| Age 85 and over | 2.4% | 1.3% | 1.9% |
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| Race | | | |
| | Janesville, WI | Butler, PA | United States |
| White | 86.8% | 87.0% | 59.4% |
| Black | 3.0% | 2.0% | 12.2% |
| Asian | 1.7% | 0.6% | 5.6% |
| Native American | 0.1% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.2% | 0.9% | 0.4% |
| Two or More Races | 2.5% | 5.8% | 3.2% |
| Hispanic | 5.7% | 3.7% | 18.4% |
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| Marriage & Family | | | |
| | Janesville, WI | Butler, PA | United States |
| Married Population | 48.7% | 33.6% | 49.8% |
| Currently Married | 47.5% | 31.8% | 48.0% |
| Married but Separated | 1.2% | 1.8% | 1.8% |
| Single Population | 51.3% | 66.4% | 50.2% |
| Never Married | 30.6% | 44.0% | 33.8% |
| Divorced | 14.4% | 16.3% | 10.8% |
| Widowed | 6.3% | 6.1% | 5.6% |
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| Household Size | 2.33 | 2.15 | 2.60 |
| Households | 27,743 | 6,036 | 124,010,992 |
| Family Households | 16,806 | 2,920 | 80,755,759 |
| Married couple, w/children | 26.5% | 23.1% | 30.5% |
| Married couple, no children | 45.2% | 35.8% | 42.9% |
| Non Family Households | 10,937 | 3,116 | 43,255,233 |
| Single householder, w/children | 18.2% | 26.2% | 15.8% |
| Single householder, no children | 10.2% | 15.0% | 10.8% |