| | Mercer Island, WA | Sierra Madre, CA | United States |
| Population | 25,506 | 11,253 | 329,725,481 |
| Female Population | 52.9% | 54.2% | 50.5% |
| Male Population | 47.1% | 45.8% | 49.5% |
| Median Age | 45.5 | 48.3 | 38.4 |
| space3 | | | |
| Population - 2010 | 22,770 | 10,916 | 309,327,143 |
| Population - 2000 | 22,036 | 10,578 | 285,036,114 |
| Population - 1990 | 20,816 | 10,762 | 251,960,433 |
| Pop. 2000 to Now | 17.6% | 0.4% | 15.6% |
| Pop. 1990 to Now | 24.5% | -1.3% | 30.8% |
| space4 | | | |
| Population Density | 3,995.9 | 3,810.7 | 93.4 |
| Land Area | 6.4 | 3.0 | 3,531,905.4 |
| Water Area | 0.0 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Mercer Island, WA | Sierra Madre, CA | United States |
| Age 0 to 4 | 3.8% | 4.5% | 5.9% |
| Age 5 to 9 | 7.3% | 5.2% | 6.1% |
| Age 10 to 14 | 8.2% | 5.8% | 6.6% |
| Age 15 to 17 | 4.3% | 2.7% | 3.9% |
| Age 18 to 20 | 2.5% | 1.9% | 4.0% |
| Age 21 to 24 | 2.9% | 1.9% | 5.2% |
| Age 25 to 34 | 8.2% | 10.4% | 13.8% |
| Age 35 to 44 | 11.8% | 12.8% | 12.9% |
| Age 45 to 54 | 16.9% | 16.4% | 12.6% |
| Age 55 to 59 | 7.9% | 10.2% | 6.7% |
| Age 60 to 64 | 6.5% | 9.0% | 6.3% |
| Age 65 to 74 | 10.4% | 9.8% | 9.6% |
| Age 75 to 84 | 5.5% | 7.4% | 4.5% |
| Age 85 and over | 3.8% | 1.8% | 1.9% |
| space1 | | | |
| Race | | | |
| | Mercer Island, WA | Sierra Madre, CA | United States |
| White | 66.8% | 61.3% | 59.4% |
| Black | 0.8% | 4.2% | 12.2% |
| Asian | 21.3% | 15.9% | 5.6% |
| Native American | 0.1% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.1% | 0.0% | 0.2% |
| Other | 0.7% | 1.1% | 0.4% |
| Two or More Races | 6.9% | 3.0% | 3.2% |
| Hispanic | 3.2% | 14.5% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Mercer Island, WA | Sierra Madre, CA | United States |
| Married Population | 64.7% | 53.6% | 49.8% |
| Currently Married | 64.3% | 53.3% | 48.0% |
| Married but Separated | 0.4% | 0.3% | 1.8% |
| Single Population | 35.3% | 46.4% | 50.2% |
| Never Married | 21.7% | 25.1% | 33.8% |
| Divorced | 7.9% | 15.9% | 10.8% |
| Widowed | 5.8% | 5.5% | 5.6% |
| space5 | | | |
| Household Size | 2.60 | 2.27 | 2.60 |
| Households | 9,758 | 4,961 | 124,010,992 |
| Family Households | 6,903 | 2,985 | 80,755,759 |
| Married couple, w/children | 42.3% | 33.9% | 30.5% |
| Married couple, no children | 48.1% | 44.0% | 42.9% |
| Non Family Households | 2,855 | 1,976 | 43,255,233 |
| Single householder, w/children | 4.6% | 6.9% | 15.8% |
| Single householder, no children | 5.0% | 15.3% | 10.8% |