| | Arnold, MO | Affton, MO | United States |
| Population | 20,808 | 18,925 | 329,725,481 |
| Female Population | 50.8% | 49.0% | 50.5% |
| Male Population | 49.2% | 51.0% | 49.5% |
| Median Age | 42.2 | 42.5 | 38.4 |
| space3 | | | |
| Population - 2010 | 20,875 | 20,115 | 309,327,143 |
| Population - 2000 | 19,965 | 20,535 | 285,036,114 |
| Population - 1990 | 18,828 | 21,106 | 251,960,433 |
| Pop. 2000 to Now | 5.9% | 4.9% | 15.6% |
| Pop. 1990 to Now | 12.3% | 2.1% | 30.8% |
| space4 | | | |
| Population Density | 1,796.4 | 4,150.2 | 93.4 |
| Land Area | 11.6 | 4.6 | 3,531,905.4 |
| Water Area | 0.4 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Arnold, MO | Affton, MO | United States |
| Age 0 to 4 | 5.7% | 4.9% | 5.9% |
| Age 5 to 9 | 5.8% | 6.0% | 6.1% |
| Age 10 to 14 | 5.4% | 3.7% | 6.6% |
| Age 15 to 17 | 4.8% | 2.5% | 3.9% |
| Age 18 to 20 | 2.8% | 2.4% | 4.0% |
| Age 21 to 24 | 2.8% | 6.0% | 5.2% |
| Age 25 to 34 | 14.9% | 14.5% | 13.8% |
| Age 35 to 44 | 11.0% | 14.5% | 12.9% |
| Age 45 to 54 | 12.6% | 12.1% | 12.6% |
| Age 55 to 59 | 8.3% | 7.3% | 6.7% |
| Age 60 to 64 | 7.5% | 7.6% | 6.3% |
| Age 65 to 74 | 10.3% | 10.5% | 9.6% |
| Age 75 to 84 | 6.0% | 4.7% | 4.5% |
| Age 85 and over | 2.2% | 3.5% | 1.9% |
| space1 | | | |
| Race | | | |
| | Arnold, MO | Affton, MO | United States |
| White | 90.4% | 87.4% | 59.4% |
| Black | 0.4% | 2.8% | 12.2% |
| Asian | 0.6% | 2.3% | 5.6% |
| Native American | 0.1% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.2% | 0.0% | 0.4% |
| Two or More Races | 6.0% | 4.0% | 3.2% |
| Hispanic | 2.4% | 3.4% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Arnold, MO | Affton, MO | United States |
| Married Population | 53.4% | 51.4% | 49.8% |
| Currently Married | 50.5% | 50.7% | 48.0% |
| Married but Separated | 2.9% | 0.7% | 1.8% |
| Single Population | 46.6% | 48.6% | 50.2% |
| Never Married | 25.8% | 29.3% | 33.8% |
| Divorced | 13.4% | 13.7% | 10.8% |
| Widowed | 7.4% | 5.6% | 5.6% |
| space5 | | | |
| Household Size | 2.46 | 2.20 | 2.60 |
| Households | 8,405 | 8,612 | 124,010,992 |
| Family Households | 5,530 | 4,837 | 80,755,759 |
| Married couple, w/children | 30.9% | 28.4% | 30.5% |
| Married couple, no children | 43.2% | 50.3% | 42.9% |
| Non Family Households | 2,875 | 3,775 | 43,255,233 |
| Single householder, w/children | 17.8% | 10.8% | 15.8% |
| Single householder, no children | 8.0% | 10.5% | 10.8% |