| | St. Marys, PA | Erie, PA | United States |
| Population | 12,771 | 95,536 | 329,725,481 |
| Female Population | 52.6% | 50.8% | 50.5% |
| Male Population | 47.4% | 49.2% | 49.5% |
| Median Age | 49.6 | 35.1 | 38.4 |
| space3 | | | |
| Population - 2010 | 13,036 | 101,719 | 309,327,143 |
| Population - 2000 | 14,502 | 103,717 | 285,036,114 |
| Population - 1990 | 5,511 | 108,718 | 251,960,433 |
| Pop. 2000 to Now | -16.2% | -8.3% | 15.6% |
| Pop. 1990 to Now | 120.6% | -12.6% | 30.8% |
| space4 | | | |
| Population Density | 128.6 | 4,992.0 | 93.4 |
| Land Area | 99.3 | 19.1 | 3,531,905.4 |
| Water Area | 0.2 | 0.2 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | St. Marys, PA | Erie, PA | United States |
| Age 0 to 4 | 5.2% | 6.4% | 5.9% |
| Age 5 to 9 | 4.8% | 5.8% | 6.1% |
| Age 10 to 14 | 4.8% | 6.2% | 6.6% |
| Age 15 to 17 | 3.0% | 3.9% | 3.9% |
| Age 18 to 20 | 2.9% | 5.5% | 4.0% |
| Age 21 to 24 | 3.3% | 6.6% | 5.2% |
| Age 25 to 34 | 10.9% | 15.5% | 13.8% |
| Age 35 to 44 | 9.9% | 10.9% | 12.9% |
| Age 45 to 54 | 12.3% | 11.3% | 12.6% |
| Age 55 to 59 | 9.0% | 6.4% | 6.7% |
| Age 60 to 64 | 8.6% | 6.6% | 6.3% |
| Age 65 to 74 | 13.8% | 8.9% | 9.6% |
| Age 75 to 84 | 8.2% | 3.9% | 4.5% |
| Age 85 and over | 3.2% | 2.2% | 1.9% |
| space1 | | | |
| Race | | | |
| | St. Marys, PA | Erie, PA | United States |
| White | 96.0% | 68.0% | 59.4% |
| Black | 0.4% | 15.2% | 12.2% |
| Asian | 0.3% | 2.8% | 5.6% |
| Native American | 0.0% | 0.3% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.6% | 0.4% |
| Two or More Races | 2.2% | 5.0% | 3.2% |
| Hispanic | 1.1% | 8.1% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | St. Marys, PA | Erie, PA | United States |
| Married Population | 49.2% | 35.4% | 49.8% |
| Currently Married | 48.5% | 32.7% | 48.0% |
| Married but Separated | 0.6% | 2.7% | 1.8% |
| Single Population | 50.8% | 64.6% | 50.2% |
| Never Married | 29.5% | 46.3% | 33.8% |
| Divorced | 12.4% | 12.4% | 10.8% |
| Widowed | 8.9% | 5.9% | 5.6% |
| space5 | | | |
| Household Size | 2.19 | 2.31 | 2.60 |
| Households | 5,691 | 38,992 | 124,010,992 |
| Family Households | 3,370 | 20,395 | 80,755,759 |
| Married couple, w/children | 15.2% | 20.7% | 30.5% |
| Married couple, no children | 59.2% | 36.3% | 42.9% |
| Non Family Households | 2,321 | 18,597 | 43,255,233 |
| Single householder, w/children | 15.8% | 30.4% | 15.8% |
| Single householder, no children | 9.8% | 12.5% | 10.8% |