| | Oak Park, CA | Belmont, CA | United States |
| Population | 13,561 | 28,109 | 329,725,481 |
| Female Population | 52.0% | 49.1% | 50.5% |
| Male Population | 48.0% | 50.9% | 49.5% |
| Median Age | 42.9 | 40.2 | 38.4 |
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| Population - 2010 | 14,044 | 25,933 | 309,327,143 |
| Population - 2000 | 2,320 | 25,123 | 285,036,114 |
| Population - 1990 | 2,412 | 24,127 | 251,960,433 |
| Pop. 2000 to Now | 519.3% | 5.6% | 15.6% |
| Pop. 1990 to Now | 495.7% | 10.0% | 30.8% |
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| Population Density | 2,561.6 | 6,065.8 | 93.4 |
| Land Area | 5.3 | 4.6 | 3,531,905.4 |
| Water Area | 0.0 | 0.0 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Oak Park, CA | Belmont, CA | United States |
| Age 0 to 4 | 6.2% | 6.0% | 5.9% |
| Age 5 to 9 | 5.3% | 6.3% | 6.1% |
| Age 10 to 14 | 8.4% | 6.5% | 6.6% |
| Age 15 to 17 | 5.3% | 4.7% | 3.9% |
| Age 18 to 20 | 3.2% | 2.5% | 4.0% |
| Age 21 to 24 | 3.8% | 2.4% | 5.2% |
| Age 25 to 34 | 6.6% | 12.7% | 13.8% |
| Age 35 to 44 | 13.7% | 16.5% | 12.9% |
| Age 45 to 54 | 15.6% | 14.8% | 12.6% |
| Age 55 to 59 | 10.1% | 7.3% | 6.7% |
| Age 60 to 64 | 6.2% | 5.6% | 6.3% |
| Age 65 to 74 | 10.4% | 8.1% | 9.6% |
| Age 75 to 84 | 2.6% | 4.0% | 4.5% |
| Age 85 and over | 2.4% | 2.6% | 1.9% |
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| Race | | | |
| | Oak Park, CA | Belmont, CA | United States |
| White | 69.1% | 49.1% | 59.4% |
| Black | 0.9% | 1.1% | 12.2% |
| Asian | 17.7% | 29.9% | 5.6% |
| Native American | 0.0% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.1% | 1.4% | 0.2% |
| Other | 0.5% | 1.1% | 0.4% |
| Two or More Races | 3.2% | 5.3% | 3.2% |
| Hispanic | 8.5% | 12.1% | 18.4% |
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| Marriage & Family | | | |
| | Oak Park, CA | Belmont, CA | United States |
| Married Population | 59.5% | 62.1% | 49.8% |
| Currently Married | 58.8% | 60.8% | 48.0% |
| Married but Separated | 0.6% | 1.2% | 1.8% |
| Single Population | 40.5% | 37.9% | 50.2% |
| Never Married | 25.6% | 25.6% | 33.8% |
| Divorced | 9.8% | 8.6% | 10.8% |
| Widowed | 5.1% | 3.6% | 5.6% |
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| Household Size | 2.70 | 2.51 | 2.60 |
| Households | 5,018 | 10,961 | 124,010,992 |
| Family Households | 3,478 | 7,662 | 80,755,759 |
| Married couple, w/children | 45.8% | 44.3% | 30.5% |
| Married couple, no children | 43.5% | 43.9% | 42.9% |
| Non Family Households | 1,540 | 3,299 | 43,255,233 |
| Single householder, w/children | 5.0% | 7.6% | 15.8% |
| Single householder, no children | 5.7% | 4.3% | 10.8% |