| | Festus, MO | Arnold, MO | United States |
| Population | 12,609 | 20,808 | 329,725,481 |
| Female Population | 53.1% | 50.8% | 50.5% |
| Male Population | 46.9% | 49.2% | 49.5% |
| Median Age | 36.5 | 42.2 | 38.4 |
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| Population - 2010 | 11,599 | 20,875 | 309,327,143 |
| Population - 2000 | 9,660 | 19,965 | 285,036,114 |
| Population - 1990 | 8,105 | 18,828 | 251,960,433 |
| Pop. 2000 to Now | 25.4% | 5.9% | 15.6% |
| Pop. 1990 to Now | 49.5% | 12.3% | 30.8% |
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| Population Density | 2,200.9 | 1,796.4 | 93.4 |
| Land Area | 5.7 | 11.6 | 3,531,905.4 |
| Water Area | 0.0 | 0.4 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Festus, MO | Arnold, MO | United States |
| Age 0 to 4 | 8.0% | 5.7% | 5.9% |
| Age 5 to 9 | 7.7% | 5.8% | 6.1% |
| Age 10 to 14 | 7.6% | 5.4% | 6.6% |
| Age 15 to 17 | 1.1% | 4.8% | 3.9% |
| Age 18 to 20 | 4.1% | 2.8% | 4.0% |
| Age 21 to 24 | 5.1% | 2.8% | 5.2% |
| Age 25 to 34 | 14.4% | 14.9% | 13.8% |
| Age 35 to 44 | 13.4% | 11.0% | 12.9% |
| Age 45 to 54 | 12.0% | 12.6% | 12.6% |
| Age 55 to 59 | 4.6% | 8.3% | 6.7% |
| Age 60 to 64 | 6.7% | 7.5% | 6.3% |
| Age 65 to 74 | 9.7% | 10.3% | 9.6% |
| Age 75 to 84 | 3.6% | 6.0% | 4.5% |
| Age 85 and over | 2.2% | 2.2% | 1.9% |
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| Race | | | |
| | Festus, MO | Arnold, MO | United States |
| White | 88.1% | 90.4% | 59.4% |
| Black | 2.4% | 0.4% | 12.2% |
| Asian | 0.7% | 0.6% | 5.6% |
| Native American | 0.1% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.2% | 0.4% |
| Two or More Races | 4.1% | 6.0% | 3.2% |
| Hispanic | 4.6% | 2.4% | 18.4% |
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| Marriage & Family | | | |
| | Festus, MO | Arnold, MO | United States |
| Married Population | 50.4% | 53.4% | 49.8% |
| Currently Married | 49.1% | 50.5% | 48.0% |
| Married but Separated | 1.3% | 2.9% | 1.8% |
| Single Population | 49.6% | 46.6% | 50.2% |
| Never Married | 25.1% | 25.8% | 33.8% |
| Divorced | 15.9% | 13.4% | 10.8% |
| Widowed | 8.5% | 7.4% | 5.6% |
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| Household Size | 2.52 | 2.46 | 2.60 |
| Households | 4,958 | 8,405 | 124,010,992 |
| Family Households | 3,320 | 5,530 | 80,755,759 |
| Married couple, w/children | 31.4% | 30.9% | 30.5% |
| Married couple, no children | 37.3% | 43.2% | 42.9% |
| Non Family Households | 1,638 | 2,875 | 43,255,233 |
| Single householder, w/children | 15.7% | 17.8% | 15.8% |
| Single householder, no children | 15.7% | 8.0% | 10.8% |