| | Hempstead, NY | Sag Harbor, NY | United States |
| Population | 791,018 | 2,480 | 329,725,481 |
| Female Population | 51.0% | 51.5% | 50.5% |
| Male Population | 49.0% | 48.6% | 49.5% |
| Median Age | 40.5 | 52.3 | 38.4 |
| space3 | | | |
| Population - 2010 | 760,725 | 2,171 | 309,327,143 |
| Population - 2000 | 755,924 | 2,313 | 285,036,114 |
| Population - 1990 | 725,639 | 2,134 | 251,960,433 |
| Pop. 2000 to Now | 1.1% | -0.9% | 15.6% |
| Pop. 1990 to Now | 5.3% | 7.4% | 30.8% |
| space4 | | | |
| Population Density | 6,670.1 | 1,374.7 | 93.4 |
| Land Area | 118.6 | 1.8 | 3,531,905.4 |
| Water Area | 73.0 | 0.5 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Hempstead, NY | Sag Harbor, NY | United States |
| Age 0 to 4 | 5.7% | 2.4% | 5.9% |
| Age 5 to 9 | 5.9% | 5.8% | 6.1% |
| Age 10 to 14 | 6.3% | 7.5% | 6.6% |
| Age 15 to 17 | 4.0% | 4.0% | 3.9% |
| Age 18 to 20 | 3.8% | 0.9% | 4.0% |
| Age 21 to 24 | 5.2% | 1.1% | 5.2% |
| Age 25 to 34 | 12.2% | 8.5% | 13.8% |
| Age 35 to 44 | 12.6% | 5.8% | 12.9% |
| Age 45 to 54 | 13.6% | 17.2% | 12.6% |
| Age 55 to 59 | 7.3% | 8.4% | 6.7% |
| Age 60 to 64 | 6.7% | 7.2% | 6.3% |
| Age 65 to 74 | 9.9% | 19.7% | 9.6% |
| Age 75 to 84 | 4.5% | 7.8% | 4.5% |
| Age 85 and over | 2.4% | 3.6% | 1.9% |
| space1 | | | |
| Race | | | |
| | Hempstead, NY | Sag Harbor, NY | United States |
| White | 52.4% | 90.2% | 59.4% |
| Black | 16.5% | 2.5% | 12.2% |
| Asian | 6.9% | 1.6% | 5.6% |
| Native American | 0.1% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.7% | 0.0% | 0.4% |
| Two or More Races | 2.5% | 1.5% | 3.2% |
| Hispanic | 21.0% | 4.2% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Hempstead, NY | Sag Harbor, NY | United States |
| Married Population | 52.8% | 51.7% | 49.8% |
| Currently Married | 51.3% | 50.7% | 48.0% |
| Married but Separated | 1.5% | 1.0% | 1.8% |
| Single Population | 47.2% | 48.3% | 50.2% |
| Never Married | 34.5% | 23.6% | 33.8% |
| Divorced | 7.0% | 12.9% | 10.8% |
| Widowed | 5.7% | 11.7% | 5.6% |
| space5 | | | |
| Household Size | 3.14 | 2.24 | 2.60 |
| Households | 248,907 | 1,095 | 124,010,992 |
| Family Households | 194,787 | 584 | 80,755,759 |
| Married couple, w/children | 35.3% | 27.4% | 30.5% |
| Married couple, no children | 40.7% | 61.3% | 42.9% |
| Non Family Households | 54,120 | 511 | 43,255,233 |
| Single householder, w/children | 10.9% | 3.6% | 15.8% |
| Single householder, no children | 13.1% | 7.7% | 10.8% |