| | Ronkonkoma, NY | Selden, NY | United States |
| Population | 18,775 | 21,265 | 329,725,481 |
| Female Population | 50.2% | 50.6% | 50.5% |
| Male Population | 49.8% | 49.4% | 49.5% |
| Median Age | 37.7 | 40.5 | 38.4 |
| space3 | | | |
| Population - 2010 | 19,779 | 19,592 | 309,327,143 |
| Population - 2000 | 20,029 | 21,861 | 285,036,114 |
| Population - 1990 | 20,391 | 20,608 | 251,960,433 |
| Pop. 2000 to Now | -8.1% | -9.9% | 15.6% |
| Pop. 1990 to Now | -9.7% | -4.5% | 30.8% |
| space4 | | | |
| Population Density | 2,400.3 | 4,921.3 | 93.4 |
| Land Area | 7.8 | 4.3 | 3,531,905.4 |
| Water Area | 0.3 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Ronkonkoma, NY | Selden, NY | United States |
| Age 0 to 4 | 6.1% | 3.8% | 5.9% |
| Age 5 to 9 | 6.1% | 5.1% | 6.1% |
| Age 10 to 14 | 4.7% | 6.2% | 6.6% |
| Age 15 to 17 | 3.2% | 4.6% | 3.9% |
| Age 18 to 20 | 2.9% | 4.5% | 4.0% |
| Age 21 to 24 | 7.2% | 4.8% | 5.2% |
| Age 25 to 34 | 15.6% | 13.5% | 13.8% |
| Age 35 to 44 | 13.5% | 12.7% | 12.9% |
| Age 45 to 54 | 14.6% | 14.7% | 12.6% |
| Age 55 to 59 | 7.8% | 6.1% | 6.7% |
| Age 60 to 64 | 4.7% | 7.3% | 6.3% |
| Age 65 to 74 | 7.9% | 9.5% | 9.6% |
| Age 75 to 84 | 3.4% | 5.5% | 4.5% |
| Age 85 and over | 2.3% | 1.9% | 1.9% |
| space1 | | | |
| Race | | | |
| | Ronkonkoma, NY | Selden, NY | United States |
| White | 65.8% | 69.0% | 59.4% |
| Black | 4.1% | 3.6% | 12.2% |
| Asian | 7.7% | 5.7% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.7% | 0.5% | 0.4% |
| Two or More Races | 0.8% | 2.7% | 3.2% |
| Hispanic | 20.9% | 18.4% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Ronkonkoma, NY | Selden, NY | United States |
| Married Population | 53.5% | 49.7% | 49.8% |
| Currently Married | 52.1% | 47.6% | 48.0% |
| Married but Separated | 1.4% | 2.0% | 1.8% |
| Single Population | 46.5% | 50.3% | 50.2% |
| Never Married | 31.9% | 35.5% | 33.8% |
| Divorced | 9.0% | 8.0% | 10.8% |
| Widowed | 5.7% | 6.9% | 5.6% |
| space5 | | | |
| Household Size | 3.05 | 3.14 | 2.60 |
| Households | 6,150 | 6,772 | 124,010,992 |
| Family Households | 4,498 | 5,335 | 80,755,759 |
| Married couple, w/children | 29.7% | 33.1% | 30.5% |
| Married couple, no children | 49.6% | 37.9% | 42.9% |
| Non Family Households | 1,652 | 1,437 | 43,255,233 |
| Single householder, w/children | 9.8% | 10.5% | 15.8% |
| Single householder, no children | 10.8% | 18.5% | 10.8% |