| | Mercer, WI | Janesville, WI | United States |
| Population | 1,385 | 65,506 | 329,725,481 |
| Female Population | 50.8% | 51.1% | 50.5% |
| Male Population | 49.2% | 48.9% | 49.5% |
| Median Age | 60.9 | 39.7 | 38.4 |
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| Population - 2010 | 1,410 | 63,632 | 309,327,143 |
| Population - 2000 | 1,732 | 59,498 | 285,036,114 |
| Population - 1990 | 1,325 | 52,133 | 251,960,433 |
| Pop. 2000 to Now | -19.9% | 8.7% | 15.6% |
| Pop. 1990 to Now | 4.7% | 24.0% | 30.8% |
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| Population Density | 8.2 | 1,924.1 | 93.4 |
| Land Area | 168.3 | 34.0 | 3,531,905.4 |
| Water Area | 16.4 | 0.6 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Mercer, WI | Janesville, WI | United States |
| Age 0 to 4 | 2.5% | 5.1% | 5.9% |
| Age 5 to 9 | 3.0% | 6.7% | 6.1% |
| Age 10 to 14 | 4.0% | 6.9% | 6.6% |
| Age 15 to 17 | 4.2% | 4.1% | 3.9% |
| Age 18 to 20 | 1.1% | 2.9% | 4.0% |
| Age 21 to 24 | 0.0% | 5.0% | 5.2% |
| Age 25 to 34 | 7.4% | 12.7% | 13.8% |
| Age 35 to 44 | 6.6% | 13.7% | 12.9% |
| Age 45 to 54 | 7.7% | 12.6% | 12.6% |
| Age 55 to 59 | 10.0% | 6.8% | 6.7% |
| Age 60 to 64 | 15.7% | 6.9% | 6.3% |
| Age 65 to 74 | 23.0% | 9.2% | 9.6% |
| Age 75 to 84 | 12.6% | 5.0% | 4.5% |
| Age 85 and over | 2.1% | 2.4% | 1.9% |
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| Race | | | |
| | Mercer, WI | Janesville, WI | United States |
| White | 95.4% | 86.8% | 59.4% |
| Black | 0.0% | 3.0% | 12.2% |
| Asian | 0.3% | 1.7% | 5.6% |
| Native American | 0.5% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.2% | 0.4% |
| Two or More Races | 0.4% | 2.5% | 3.2% |
| Hispanic | 3.5% | 5.7% | 18.4% |
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| Marriage & Family | | | |
| | Mercer, WI | Janesville, WI | United States |
| Married Population | 62.3% | 48.7% | 49.8% |
| Currently Married | 62.3% | 47.5% | 48.0% |
| Married but Separated | 0.0% | 1.2% | 1.8% |
| Single Population | 37.7% | 51.3% | 50.2% |
| Never Married | 17.9% | 30.6% | 33.8% |
| Divorced | 15.9% | 14.4% | 10.8% |
| Widowed | 3.9% | 6.3% | 5.6% |
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| Household Size | 2.16 | 2.33 | 2.60 |
| Households | 641 | 27,743 | 124,010,992 |
| Family Households | 419 | 16,806 | 80,755,759 |
| Married couple, w/children | 8.6% | 26.5% | 30.5% |
| Married couple, no children | 80.7% | 45.2% | 42.9% |
| Non Family Households | 222 | 10,937 | 43,255,233 |
| Single householder, w/children | 7.4% | 18.2% | 15.8% |
| Single householder, no children | 3.3% | 10.2% | 10.8% |