| | Diamond Bar, CA | Philadelphia, PA | United States |
| Population | 55,339 | 1,596,865 | 329,725,481 |
| Female Population | 51.7% | 52.4% | 50.5% |
| Male Population | 48.3% | 47.6% | 49.5% |
| Median Age | 43.7 | 34.8 | 38.4 |
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| Population - 2010 | 55,559 | 1,528,303 | 309,327,143 |
| Population - 2000 | 56,287 | 1,517,550 | 285,036,114 |
| Population - 1990 | 53,672 | 1,585,577 | 251,960,433 |
| Pop. 2000 to Now | -2.5% | 4.0% | 15.6% |
| Pop. 1990 to Now | 2.3% | -0.5% | 30.8% |
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| Population Density | 3,721.5 | 11,896.2 | 93.4 |
| Land Area | 14.9 | 134.2 | 3,531,905.4 |
| Water Area | 0.0 | 8.5 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Diamond Bar, CA | Philadelphia, PA | United States |
| Age 0 to 4 | 5.3% | 6.4% | 5.9% |
| Age 5 to 9 | 5.4% | 5.9% | 6.1% |
| Age 10 to 14 | 6.2% | 6.2% | 6.6% |
| Age 15 to 17 | 3.4% | 3.3% | 3.9% |
| Age 18 to 20 | 2.8% | 4.1% | 4.0% |
| Age 21 to 24 | 4.1% | 5.5% | 5.2% |
| Age 25 to 34 | 12.1% | 18.9% | 13.8% |
| Age 35 to 44 | 12.4% | 12.9% | 12.9% |
| Age 45 to 54 | 14.6% | 11.3% | 12.6% |
| Age 55 to 59 | 8.2% | 5.8% | 6.7% |
| Age 60 to 64 | 7.2% | 5.9% | 6.3% |
| Age 65 to 74 | 12.6% | 8.1% | 9.6% |
| Age 75 to 84 | 4.0% | 3.9% | 4.5% |
| Age 85 and over | 1.8% | 1.7% | 1.9% |
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| Race | | | |
| | Diamond Bar, CA | Philadelphia, PA | United States |
| White | 14.1% | 33.8% | 59.4% |
| Black | 4.0% | 39.6% | 12.2% |
| Asian | 60.7% | 7.4% | 5.6% |
| Native American | 0.2% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.2% | 0.0% | 0.2% |
| Other | 0.4% | 0.4% | 0.4% |
| Two or More Races | 2.7% | 3.2% | 3.2% |
| Hispanic | 17.8% | 15.4% | 18.4% |
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| Marriage & Family | | | |
| | Diamond Bar, CA | Philadelphia, PA | United States |
| Married Population | 57.8% | 34.6% | 49.8% |
| Currently Married | 57.2% | 31.5% | 48.0% |
| Married but Separated | 0.7% | 3.0% | 1.8% |
| Single Population | 42.2% | 65.4% | 50.2% |
| Never Married | 29.5% | 50.5% | 33.8% |
| Divorced | 7.6% | 9.2% | 10.8% |
| Widowed | 5.1% | 5.7% | 5.6% |
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| Household Size | 3.10 | 2.40 | 2.60 |
| Households | 17,788 | 646,608 | 124,010,992 |
| Family Households | 14,446 | 347,858 | 80,755,759 |
| Married couple, w/children | 32.0% | 21.3% | 30.5% |
| Married couple, no children | 47.1% | 30.6% | 42.9% |
| Non Family Households | 3,342 | 298,750 | 43,255,233 |
| Single householder, w/children | 8.7% | 28.8% | 15.8% |
| Single householder, no children | 12.2% | 19.4% | 10.8% |