| | Arnold, CA | Arnold, MD | United States |
| Population | 2,109 | 25,056 | 329,725,481 |
| Female Population | 49.4% | 51.2% | 50.5% |
| Male Population | 50.6% | 48.8% | 49.5% |
| Median Age | 63.3 | 38.2 | 38.4 |
| space3 | | | |
| Population - 2010 | 3,439 | 22,483 | 309,327,143 |
| Population - 2000 | 4,218 | 23,422 | 285,036,114 |
| Population - 1990 | 3,788 | 20,261 | 251,960,433 |
| Pop. 2000 to Now | -31.7% | 1.8% | 15.6% |
| Pop. 1990 to Now | -23.9% | 17.7% | 30.8% |
| space4 | | | |
| Population Density | 142.6 | 2,316.1 | 93.4 |
| Land Area | 14.8 | 10.8 | 3,531,905.4 |
| Water Area | 0.1 | 2.7 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Arnold, CA | Arnold, MD | United States |
| Age 0 to 4 | 1.8% | 5.9% | 5.9% |
| Age 5 to 9 | 2.4% | 8.0% | 6.1% |
| Age 10 to 14 | 1.8% | 8.4% | 6.6% |
| Age 15 to 17 | 0.0% | 5.7% | 3.9% |
| Age 18 to 20 | 0.6% | 3.5% | 4.0% |
| Age 21 to 24 | 1.9% | 3.3% | 5.2% |
| Age 25 to 34 | 8.3% | 8.7% | 13.8% |
| Age 35 to 44 | 8.4% | 15.7% | 12.9% |
| Age 45 to 54 | 8.5% | 13.4% | 12.6% |
| Age 55 to 59 | 8.9% | 7.9% | 6.7% |
| Age 60 to 64 | 11.2% | 5.0% | 6.3% |
| Age 65 to 74 | 31.7% | 9.3% | 9.6% |
| Age 75 to 84 | 11.1% | 3.6% | 4.5% |
| Age 85 and over | 3.4% | 1.5% | 1.9% |
| space1 | | | |
| Race | | | |
| | Arnold, CA | Arnold, MD | United States |
| White | 87.6% | 77.1% | 59.4% |
| Black | 0.0% | 5.1% | 12.2% |
| Asian | 4.2% | 3.5% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.1% | 0.4% |
| Two or More Races | 0.6% | 5.4% | 3.2% |
| Hispanic | 7.7% | 8.8% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Arnold, CA | Arnold, MD | United States |
| Married Population | 66.5% | 61.4% | 49.8% |
| Currently Married | 63.1% | 59.4% | 48.0% |
| Married but Separated | 3.5% | 2.0% | 1.8% |
| Single Population | 33.5% | 38.6% | 50.2% |
| Never Married | 13.6% | 26.5% | 33.8% |
| Divorced | 14.3% | 9.1% | 10.8% |
| Widowed | 5.6% | 3.1% | 5.6% |
| space5 | | | |
| Household Size | 2.07 | 2.88 | 2.60 |
| Households | 1,019 | 8,633 | 124,010,992 |
| Family Households | 653 | 6,788 | 80,755,759 |
| Married couple, w/children | 3.8% | 42.1% | 30.5% |
| Married couple, no children | 85.1% | 38.8% | 42.9% |
| Non Family Households | 366 | 1,845 | 43,255,233 |
| Single householder, w/children | 6.0% | 11.2% | 15.8% |
| Single householder, no children | 5.1% | 7.9% | 10.8% |