| | St. Louis, MO | Atlantic Beach, FL | United States |
| Population | 302,787 | 13,404 | 329,725,481 |
| Female Population | 51.3% | 52.9% | 50.5% |
| Male Population | 48.7% | 47.1% | 49.5% |
| Median Age | 36.3 | 49.7 | 38.4 |
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| Population - 2010 | 319,367 | 12,663 | 309,327,143 |
| Population - 2000 | 348,189 | 13,368 | 285,036,114 |
| Population - 1990 | 396,685 | 11,636 | 251,960,433 |
| Pop. 2000 to Now | -14.5% | 3.6% | 15.6% |
| Pop. 1990 to Now | -25.0% | 19.0% | 30.8% |
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| Population Density | 4,903.9 | 3,844.0 | 93.4 |
| Land Area | 61.7 | 3.5 | 3,531,905.4 |
| Water Area | 4.3 | 9.5 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | St. Louis, MO | Atlantic Beach, FL | United States |
| Age 0 to 4 | 6.1% | 3.6% | 5.9% |
| Age 5 to 9 | 4.8% | 4.4% | 6.1% |
| Age 10 to 14 | 5.3% | 5.7% | 6.6% |
| Age 15 to 17 | 2.8% | 3.6% | 3.9% |
| Age 18 to 20 | 3.5% | 1.8% | 4.0% |
| Age 21 to 24 | 5.4% | 4.5% | 5.2% |
| Age 25 to 34 | 19.9% | 10.4% | 13.8% |
| Age 35 to 44 | 13.7% | 10.5% | 12.9% |
| Age 45 to 54 | 11.5% | 14.3% | 12.6% |
| Age 55 to 59 | 6.4% | 9.9% | 6.7% |
| Age 60 to 64 | 6.8% | 9.0% | 6.3% |
| Age 65 to 74 | 8.8% | 12.5% | 9.6% |
| Age 75 to 84 | 3.5% | 6.0% | 4.5% |
| Age 85 and over | 1.7% | 3.8% | 1.9% |
| space1 | | | |
| Race | | | |
| | St. Louis, MO | Atlantic Beach, FL | United States |
| White | 44.3% | 80.2% | 59.4% |
| Black | 44.6% | 6.0% | 12.2% |
| Asian | 3.4% | 3.1% | 5.6% |
| Native American | 0.2% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.1% | 0.0% | 0.2% |
| Other | 0.4% | 1.1% | 0.4% |
| Two or More Races | 2.8% | 3.6% | 3.2% |
| Hispanic | 4.2% | 6.0% | 18.4% |
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| Marriage & Family | | | |
| | St. Louis, MO | Atlantic Beach, FL | United States |
| Married Population | 33.6% | 53.6% | 49.8% |
| Currently Married | 30.7% | 52.5% | 48.0% |
| Married but Separated | 2.9% | 1.1% | 1.8% |
| Single Population | 66.4% | 46.4% | 50.2% |
| Never Married | 48.8% | 27.3% | 33.8% |
| Divorced | 12.3% | 12.5% | 10.8% |
| Widowed | 5.2% | 6.7% | 5.6% |
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| Household Size | 2.08 | 2.24 | 2.60 |
| Households | 141,161 | 5,960 | 124,010,992 |
| Family Households | 62,559 | 3,623 | 80,755,759 |
| Married couple, w/children | 20.7% | 21.9% | 30.5% |
| Married couple, no children | 36.1% | 55.0% | 42.9% |
| Non Family Households | 78,602 | 2,337 | 43,255,233 |
| Single householder, w/children | 26.9% | 9.9% | 15.8% |
| Single householder, no children | 16.4% | 13.2% | 10.8% |