| | Portsmouth, NH | Atascadero, CA | United States |
| Population | 21,897 | 29,771 | 329,725,481 |
| Female Population | 52.5% | 50.1% | 50.5% |
| Male Population | 47.6% | 49.9% | 49.5% |
| Median Age | 42.2 | 40 | 38.4 |
| space3 | | | |
| Population - 2010 | 20,770 | 28,425 | 309,327,143 |
| Population - 2000 | 20,784 | 26,411 | 285,036,114 |
| Population - 1990 | 25,925 | 23,138 | 251,960,433 |
| Pop. 2000 to Now | 3.7% | 14.6% | 15.6% |
| Pop. 1990 to Now | -16.9% | 30.8% | 30.8% |
| space4 | | | |
| Population Density | 1,398.4 | 1,141.3 | 93.4 |
| Land Area | 15.7 | 26.1 | 3,531,905.4 |
| Water Area | 1.2 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Portsmouth, NH | Atascadero, CA | United States |
| Age 0 to 4 | 3.5% | 5.7% | 5.9% |
| Age 5 to 9 | 4.9% | 5.5% | 6.1% |
| Age 10 to 14 | 3.9% | 5.8% | 6.6% |
| Age 15 to 17 | 2.9% | 3.3% | 3.9% |
| Age 18 to 20 | 1.8% | 2.7% | 4.0% |
| Age 21 to 24 | 4.9% | 5.9% | 5.2% |
| Age 25 to 34 | 17.5% | 13.9% | 13.8% |
| Age 35 to 44 | 13.6% | 11.9% | 12.9% |
| Age 45 to 54 | 13.0% | 12.6% | 12.6% |
| Age 55 to 59 | 6.5% | 7.3% | 6.7% |
| Age 60 to 64 | 7.3% | 8.4% | 6.3% |
| Age 65 to 74 | 12.0% | 10.5% | 9.6% |
| Age 75 to 84 | 5.9% | 5.2% | 4.5% |
| Age 85 and over | 2.4% | 1.3% | 1.9% |
| space1 | | | |
| Race | | | |
| | Portsmouth, NH | Atascadero, CA | United States |
| White | 87.3% | 74.3% | 59.4% |
| Black | 1.2% | 0.6% | 12.2% |
| Asian | 5.0% | 1.6% | 5.6% |
| Native American | 0.0% | 0.4% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 1.4% | 0.5% | 0.4% |
| Two or More Races | 2.9% | 4.3% | 3.2% |
| Hispanic | 2.3% | 18.3% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Portsmouth, NH | Atascadero, CA | United States |
| Married Population | 48.6% | 54.8% | 49.8% |
| Currently Married | 48.0% | 53.1% | 48.0% |
| Married but Separated | 0.7% | 1.7% | 1.8% |
| Single Population | 51.4% | 45.2% | 50.2% |
| Never Married | 33.7% | 29.9% | 33.8% |
| Divorced | 12.3% | 10.7% | 10.8% |
| Widowed | 5.4% | 4.6% | 5.6% |
| space5 | | | |
| Household Size | 2.05 | 2.51 | 2.60 |
| Households | 10,462 | 11,733 | 124,010,992 |
| Family Households | 5,348 | 8,042 | 80,755,759 |
| Married couple, w/children | 23.9% | 29.3% | 30.5% |
| Married couple, no children | 55.7% | 47.9% | 42.9% |
| Non Family Households | 5,114 | 3,691 | 43,255,233 |
| Single householder, w/children | 10.9% | 12.0% | 15.8% |
| Single householder, no children | 9.6% | 10.7% | 10.8% |