| | Greenlawn, NY | Rockville Centre, NY | United States |
| Population | 15,115 | 25,768 | 329,725,481 |
| Female Population | 52.1% | 52.7% | 50.5% |
| Male Population | 47.9% | 47.3% | 49.5% |
| Median Age | 39.7 | 43.8 | 38.4 |
| space3 | | | |
| Population - 2010 | 12,885 | 24,075 | 309,327,143 |
| Population - 2000 | 13,286 | 24,568 | 285,036,114 |
| Population - 1990 | 13,208 | 24,727 | 251,960,433 |
| Pop. 2000 to Now | 3.0% | -0.7% | 15.6% |
| Pop. 1990 to Now | 3.6% | -1.3% | 30.8% |
| space4 | | | |
| Population Density | 4,058.8 | 7,926.2 | 93.4 |
| Land Area | 3.7 | 3.3 | 3,531,905.4 |
| Water Area | 0.0 | 0.1 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Greenlawn, NY | Rockville Centre, NY | United States |
| Age 0 to 4 | 4.3% | 5.3% | 5.9% |
| Age 5 to 9 | 7.9% | 7.4% | 6.1% |
| Age 10 to 14 | 8.8% | 5.7% | 6.6% |
| Age 15 to 17 | 6.0% | 4.5% | 3.9% |
| Age 18 to 20 | 2.5% | 3.2% | 4.0% |
| Age 21 to 24 | 6.4% | 2.7% | 5.2% |
| Age 25 to 34 | 7.8% | 9.2% | 13.8% |
| Age 35 to 44 | 11.8% | 13.4% | 12.9% |
| Age 45 to 54 | 15.4% | 13.9% | 12.6% |
| Age 55 to 59 | 6.7% | 7.3% | 6.7% |
| Age 60 to 64 | 6.3% | 7.8% | 6.3% |
| Age 65 to 74 | 8.9% | 11.4% | 9.6% |
| Age 75 to 84 | 3.5% | 4.7% | 4.5% |
| Age 85 and over | 3.6% | 3.6% | 1.9% |
| space1 | | | |
| Race | | | |
| | Greenlawn, NY | Rockville Centre, NY | United States |
| White | 61.4% | 78.9% | 59.4% |
| Black | 12.9% | 5.0% | 12.2% |
| Asian | 3.4% | 5.1% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.8% | 0.6% | 0.4% |
| Two or More Races | 3.3% | 1.7% | 3.2% |
| Hispanic | 18.2% | 8.8% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Greenlawn, NY | Rockville Centre, NY | United States |
| Married Population | 51.4% | 57.7% | 49.8% |
| Currently Married | 49.7% | 56.5% | 48.0% |
| Married but Separated | 1.7% | 1.2% | 1.8% |
| Single Population | 48.6% | 42.3% | 50.2% |
| Never Married | 33.0% | 29.9% | 33.8% |
| Divorced | 8.3% | 6.3% | 10.8% |
| Widowed | 7.4% | 6.1% | 5.6% |
| space5 | | | |
| Household Size | 3.27 | 2.53 | 2.60 |
| Households | 4,556 | 10,070 | 124,010,992 |
| Family Households | 3,381 | 6,737 | 80,755,759 |
| Married couple, w/children | 40.4% | 40.2% | 30.5% |
| Married couple, no children | 36.3% | 43.0% | 42.9% |
| Non Family Households | 1,175 | 3,333 | 43,255,233 |
| Single householder, w/children | 12.2% | 9.0% | 15.8% |
| Single householder, no children | 11.2% | 7.7% | 10.8% |