| | Johnston, RI | Allentown, PA | United States |
| Population | 29,550 | 125,250 | 329,725,481 |
| Female Population | 51.6% | 51.2% | 50.5% |
| Male Population | 48.4% | 48.8% | 49.5% |
| Median Age | 45.1 | 31.9 | 38.4 |
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| Population - 2010 | 28,877 | 118,040 | 309,327,143 |
| Population - 2000 | 28,195 | 106,632 | 285,036,114 |
| Population - 1990 | 26,542 | 105,090 | 251,960,433 |
| Pop. 2000 to Now | 4.2% | 13.9% | 15.6% |
| Pop. 1990 to Now | 10.6% | 15.6% | 30.8% |
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| Population Density | 1,261.2 | 7,137.6 | 93.4 |
| Land Area | 23.4 | 17.5 | 3,531,905.4 |
| Water Area | 0.9 | 0.5 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Johnston, RI | Allentown, PA | United States |
| Age 0 to 4 | 4.5% | 7.7% | 5.9% |
| Age 5 to 9 | 5.6% | 6.7% | 6.1% |
| Age 10 to 14 | 5.1% | 7.4% | 6.6% |
| Age 15 to 17 | 2.9% | 3.9% | 3.9% |
| Age 18 to 20 | 2.6% | 5.8% | 4.0% |
| Age 21 to 24 | 3.3% | 6.5% | 5.2% |
| Age 25 to 34 | 13.3% | 16.3% | 13.8% |
| Age 35 to 44 | 12.6% | 12.0% | 12.9% |
| Age 45 to 54 | 14.5% | 10.7% | 12.6% |
| Age 55 to 59 | 5.3% | 5.7% | 6.7% |
| Age 60 to 64 | 8.8% | 5.0% | 6.3% |
| Age 65 to 74 | 11.4% | 7.2% | 9.6% |
| Age 75 to 84 | 6.6% | 3.3% | 4.5% |
| Age 85 and over | 3.4% | 1.8% | 1.9% |
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| Race | | | |
| | Johnston, RI | Allentown, PA | United States |
| White | 80.0% | 29.9% | 59.4% |
| Black | 4.6% | 10.3% | 12.2% |
| Asian | 1.1% | 3.0% | 5.6% |
| Native American | 0.0% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.3% | 0.3% | 0.4% |
| Two or More Races | 0.8% | 1.7% | 3.2% |
| Hispanic | 13.3% | 54.5% | 18.4% |
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| Marriage & Family | | | |
| | Johnston, RI | Allentown, PA | United States |
| Married Population | 45.5% | 38.5% | 49.8% |
| Currently Married | 44.8% | 34.6% | 48.0% |
| Married but Separated | 0.8% | 3.9% | 1.8% |
| Single Population | 54.5% | 61.5% | 50.2% |
| Never Married | 34.2% | 46.7% | 33.8% |
| Divorced | 11.0% | 9.8% | 10.8% |
| Widowed | 9.2% | 5.0% | 5.6% |
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| Household Size | 2.53 | 2.65 | 2.60 |
| Households | 11,482 | 45,195 | 124,010,992 |
| Family Households | 7,350 | 27,940 | 80,755,759 |
| Married couple, w/children | 29.2% | 25.0% | 30.5% |
| Married couple, no children | 40.3% | 28.9% | 42.9% |
| Non Family Households | 4,132 | 17,255 | 43,255,233 |
| Single householder, w/children | 14.0% | 31.5% | 15.8% |
| Single householder, no children | 16.5% | 14.6% | 10.8% |