| | Katy, TX | St. Louis, MO | United States |
| Population | 21,926 | 302,787 | 329,725,481 |
| Female Population | 50.4% | 51.3% | 50.5% |
| Male Population | 49.6% | 48.7% | 49.5% |
| Median Age | 39.7 | 36.3 | 38.4 |
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| Population - 2010 | 14,166 | 319,367 | 309,327,143 |
| Population - 2000 | 11,775 | 348,189 | 285,036,114 |
| Population - 1990 | 8,005 | 396,685 | 251,960,433 |
| Pop. 2000 to Now | 102.9% | -14.5% | 15.6% |
| Pop. 1990 to Now | 198.4% | -25.0% | 30.8% |
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| Population Density | 1,508.5 | 4,903.9 | 93.4 |
| Land Area | 14.5 | 61.7 | 3,531,905.4 |
| Water Area | 0.0 | 4.3 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Katy, TX | St. Louis, MO | United States |
| Age 0 to 4 | 6.9% | 6.1% | 5.9% |
| Age 5 to 9 | 9.2% | 4.8% | 6.1% |
| Age 10 to 14 | 8.3% | 5.3% | 6.6% |
| Age 15 to 17 | 3.5% | 2.8% | 3.9% |
| Age 18 to 20 | 1.5% | 3.5% | 4.0% |
| Age 21 to 24 | 3.9% | 5.4% | 5.2% |
| Age 25 to 34 | 11.0% | 19.9% | 13.8% |
| Age 35 to 44 | 14.1% | 13.7% | 12.9% |
| Age 45 to 54 | 16.7% | 11.5% | 12.6% |
| Age 55 to 59 | 5.6% | 6.4% | 6.7% |
| Age 60 to 64 | 6.5% | 6.8% | 6.3% |
| Age 65 to 74 | 8.2% | 8.8% | 9.6% |
| Age 75 to 84 | 3.4% | 3.5% | 4.5% |
| Age 85 and over | 1.3% | 1.7% | 1.9% |
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| Race | | | |
| | Katy, TX | St. Louis, MO | United States |
| White | 53.2% | 44.3% | 59.4% |
| Black | 11.2% | 44.6% | 12.2% |
| Asian | 6.3% | 3.4% | 5.6% |
| Native American | 0.0% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.1% | 0.2% |
| Other | 1.1% | 0.4% | 0.4% |
| Two or More Races | 4.6% | 2.8% | 3.2% |
| Hispanic | 23.7% | 4.2% | 18.4% |
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| Marriage & Family | | | |
| | Katy, TX | St. Louis, MO | United States |
| Married Population | 65.9% | 33.6% | 49.8% |
| Currently Married | 63.2% | 30.7% | 48.0% |
| Married but Separated | 2.6% | 2.9% | 1.8% |
| Single Population | 34.1% | 66.4% | 50.2% |
| Never Married | 19.8% | 48.8% | 33.8% |
| Divorced | 9.8% | 12.3% | 10.8% |
| Widowed | 4.5% | 5.2% | 5.6% |
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| Household Size | 3.07 | 2.08 | 2.60 |
| Households | 7,095 | 141,161 | 124,010,992 |
| Family Households | 5,653 | 62,559 | 80,755,759 |
| Married couple, w/children | 40.4% | 20.7% | 30.5% |
| Married couple, no children | 45.5% | 36.1% | 42.9% |
| Non Family Households | 1,442 | 78,602 | 43,255,233 |
| Single householder, w/children | 7.1% | 26.9% | 15.8% |
| Single householder, no children | 7.0% | 16.4% | 10.8% |