| | Onset, MA | Rockland, ME | United States |
| Population | 1,138 | 6,977 | 329,725,481 |
| Female Population | 50.9% | 53.1% | 50.5% |
| Male Population | 49.1% | 46.9% | 49.5% |
| Median Age | 53.6 | 45.4 | 38.4 |
| space3 | | | |
| Population - 2010 | 926 | 7,295 | 309,327,143 |
| Population - 2000 | 1,292 | 7,609 | 285,036,114 |
| Population - 1990 | 1,461 | 7,972 | 251,960,433 |
| Pop. 2000 to Now | -35.2% | -5.6% | 15.6% |
| Pop. 1990 to Now | -42.7% | -9.9% | 30.8% |
| space4 | | | |
| Population Density | 1,055.7 | 543.3 | 93.4 |
| Land Area | 1.1 | 12.8 | 3,531,905.4 |
| Water Area | 0.2 | 2.2 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Onset, MA | Rockland, ME | United States |
| Age 0 to 4 | 0.0% | 3.8% | 5.9% |
| Age 5 to 9 | 8.8% | 5.3% | 6.1% |
| Age 10 to 14 | 0.0% | 6.4% | 6.6% |
| Age 15 to 17 | 0.0% | 3.5% | 3.9% |
| Age 18 to 20 | 0.0% | 2.7% | 4.0% |
| Age 21 to 24 | 5.7% | 4.4% | 5.2% |
| Age 25 to 34 | 4.2% | 13.6% | 13.8% |
| Age 35 to 44 | 12.8% | 9.7% | 12.9% |
| Age 45 to 54 | 24.3% | 8.3% | 12.6% |
| Age 55 to 59 | 6.0% | 9.1% | 6.7% |
| Age 60 to 64 | 8.3% | 11.2% | 6.3% |
| Age 65 to 74 | 14.6% | 12.2% | 9.6% |
| Age 75 to 84 | 8.3% | 7.1% | 4.5% |
| Age 85 and over | 7.0% | 2.8% | 1.9% |
| space1 | | | |
| Race | | | |
| | Onset, MA | Rockland, ME | United States |
| White | 60.1% | 95.4% | 59.4% |
| Black | 10.5% | 0.0% | 12.2% |
| Asian | 0.0% | 0.9% | 5.6% |
| Native American | 0.0% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 16.2% | 0.0% | 0.4% |
| Two or More Races | 13.3% | 2.6% | 3.2% |
| Hispanic | 0.0% | 0.9% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Onset, MA | Rockland, ME | United States |
| Married Population | 31.9% | 38.6% | 49.8% |
| Currently Married | 31.9% | 37.2% | 48.0% |
| Married but Separated | 0.0% | 1.5% | 1.8% |
| Single Population | 68.1% | 61.4% | 50.2% |
| Never Married | 37.4% | 33.6% | 33.8% |
| Divorced | 14.3% | 20.6% | 10.8% |
| Widowed | 16.5% | 7.2% | 5.6% |
| space5 | | | |
| Household Size | 1.84 | 2.01 | 2.60 |
| Households | 614 | 3,367 | 124,010,992 |
| Family Households | 285 | 1,437 | 80,755,759 |
| Married couple, w/children | 0.0% | 21.5% | 30.5% |
| Married couple, no children | 55.4% | 53.0% | 42.9% |
| Non Family Households | 329 | 1,930 | 43,255,233 |
| Single householder, w/children | 17.9% | 21.2% | 15.8% |
| Single householder, no children | 26.7% | 4.4% | 10.8% |