| | Northumberland, PA | Dillsburg, PA | United States |
| Population | 3,895 | 2,635 | 329,725,481 |
| Female Population | 50.5% | 54.8% | 50.5% |
| Male Population | 49.5% | 45.2% | 49.5% |
| Median Age | 43 | 33 | 38.4 |
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| Population - 2010 | 3,776 | 2,585 | 309,327,143 |
| Population - 2000 | 3,714 | 2,063 | 285,036,114 |
| Population - 1990 | 3,860 | 1,925 | 251,960,433 |
| Pop. 2000 to Now | -3.9% | 25.0% | 15.6% |
| Pop. 1990 to Now | -7.5% | 34.0% | 30.8% |
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| Population Density | 2,584.6 | 3,289.6 | 93.4 |
| Land Area | 1.5 | 0.8 | 3,531,905.4 |
| Water Area | 0.0 | 0.0 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Northumberland, PA | Dillsburg, PA | United States |
| Age 0 to 4 | 3.0% | 6.4% | 5.9% |
| Age 5 to 9 | 7.3% | 6.9% | 6.1% |
| Age 10 to 14 | 9.3% | 10.6% | 6.6% |
| Age 15 to 17 | 2.6% | 5.7% | 3.9% |
| Age 18 to 20 | 2.7% | 0.8% | 4.0% |
| Age 21 to 24 | 1.4% | 4.6% | 5.2% |
| Age 25 to 34 | 10.3% | 17.1% | 13.8% |
| Age 35 to 44 | 14.9% | 15.4% | 12.9% |
| Age 45 to 54 | 12.2% | 10.1% | 12.6% |
| Age 55 to 59 | 7.8% | 7.4% | 6.7% |
| Age 60 to 64 | 4.7% | 3.4% | 6.3% |
| Age 65 to 74 | 11.0% | 6.2% | 9.6% |
| Age 75 to 84 | 9.2% | 3.6% | 4.5% |
| Age 85 and over | 3.5% | 1.7% | 1.9% |
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| Race | | | |
| | Northumberland, PA | Dillsburg, PA | United States |
| White | 90.8% | 88.4% | 59.4% |
| Black | 1.0% | 1.1% | 12.2% |
| Asian | 0.0% | 0.7% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 1.9% | 0.0% | 0.4% |
| Two or More Races | 2.0% | 2.9% | 3.2% |
| Hispanic | 4.3% | 6.9% | 18.4% |
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| Marriage & Family | | | |
| | Northumberland, PA | Dillsburg, PA | United States |
| Married Population | 57.8% | 48.5% | 49.8% |
| Currently Married | 55.5% | 46.5% | 48.0% |
| Married but Separated | 2.3% | 2.0% | 1.8% |
| Single Population | 42.2% | 51.5% | 50.2% |
| Never Married | 22.5% | 28.0% | 33.8% |
| Divorced | 13.7% | 20.2% | 10.8% |
| Widowed | 6.0% | 3.2% | 5.6% |
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| Household Size | 2.25 | 2.66 | 2.60 |
| Households | 1,717 | 991 | 124,010,992 |
| Family Households | 1,074 | 651 | 80,755,759 |
| Married couple, w/children | 25.4% | 36.9% | 30.5% |
| Married couple, no children | 50.7% | 29.5% | 42.9% |
| Non Family Households | 643 | 340 | 43,255,233 |
| Single householder, w/children | 20.7% | 22.6% | 15.8% |
| Single householder, no children | 3.3% | 11.1% | 10.8% |