| | Arnold, MD | Portland, ME | United States |
| Population | 25,056 | 68,063 | 329,725,481 |
| Female Population | 51.2% | 51.2% | 50.5% |
| Male Population | 48.8% | 48.9% | 49.5% |
| Median Age | 38.2 | 36.9 | 38.4 |
| space3 | | | |
| Population - 2010 | 22,483 | 66,089 | 309,327,143 |
| Population - 2000 | 23,422 | 64,249 | 285,036,114 |
| Population - 1990 | 20,261 | 64,358 | 251,960,433 |
| Pop. 2000 to Now | 1.8% | 4.0% | 15.6% |
| Pop. 1990 to Now | 17.7% | 3.8% | 30.8% |
| space4 | | | |
| Population Density | 2,316.1 | 3,155.6 | 93.4 |
| Land Area | 10.8 | 21.6 | 3,531,905.4 |
| Water Area | 2.7 | 47.9 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Arnold, MD | Portland, ME | United States |
| Age 0 to 4 | 5.9% | 4.9% | 5.9% |
| Age 5 to 9 | 8.0% | 4.4% | 6.1% |
| Age 10 to 14 | 8.4% | 4.5% | 6.6% |
| Age 15 to 17 | 5.7% | 2.1% | 3.9% |
| Age 18 to 20 | 3.5% | 2.7% | 4.0% |
| Age 21 to 24 | 3.3% | 6.4% | 5.2% |
| Age 25 to 34 | 8.7% | 21.3% | 13.8% |
| Age 35 to 44 | 15.7% | 14.5% | 12.9% |
| Age 45 to 54 | 13.4% | 11.2% | 12.6% |
| Age 55 to 59 | 7.9% | 6.9% | 6.7% |
| Age 60 to 64 | 5.0% | 5.7% | 6.3% |
| Age 65 to 74 | 9.3% | 9.1% | 9.6% |
| Age 75 to 84 | 3.6% | 4.0% | 4.5% |
| Age 85 and over | 1.5% | 2.2% | 1.9% |
| space1 | | | |
| Race | | | |
| | Arnold, MD | Portland, ME | United States |
| White | 77.1% | 81.7% | 59.4% |
| Black | 5.1% | 8.9% | 12.2% |
| Asian | 3.5% | 4.0% | 5.6% |
| Native American | 0.0% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.1% | 0.1% | 0.4% |
| Two or More Races | 5.4% | 2.8% | 3.2% |
| Hispanic | 8.8% | 2.4% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Arnold, MD | Portland, ME | United States |
| Married Population | 61.4% | 39.6% | 49.8% |
| Currently Married | 59.4% | 38.6% | 48.0% |
| Married but Separated | 2.0% | 1.0% | 1.8% |
| Single Population | 38.6% | 60.4% | 50.2% |
| Never Married | 26.5% | 44.4% | 33.8% |
| Divorced | 9.1% | 11.4% | 10.8% |
| Widowed | 3.1% | 4.6% | 5.6% |
| space5 | | | |
| Household Size | 2.88 | 2.12 | 2.60 |
| Households | 8,633 | 31,237 | 124,010,992 |
| Family Households | 6,788 | 13,490 | 80,755,759 |
| Married couple, w/children | 42.1% | 28.1% | 30.5% |
| Married couple, no children | 38.8% | 49.2% | 42.9% |
| Non Family Households | 1,845 | 17,747 | 43,255,233 |
| Single householder, w/children | 11.2% | 13.7% | 15.8% |
| Single householder, no children | 7.9% | 9.0% | 10.8% |