| | Mount Sidney, VA | Brambleton, VA | United States |
| Population | 604 | 21,986 | 329,725,481 |
| Female Population | 52.7% | 49.1% | 50.5% |
| Male Population | 47.4% | 51.0% | 49.5% |
| Median Age | 31.7 | 36.6 | 38.4 |
| space3 | | | |
| Population - 2010 | 619 | 8,595 | 309,327,143 |
| Population - 2000 | 533 | 165 | 285,036,114 |
| Population - 1990 | 499 | 85 | 251,960,433 |
| Pop. 2000 to Now | -2.6% | 13,510.9% | 15.6% |
| Pop. 1990 to Now | 4.0% | 26,321.2% | 30.8% |
| space4 | | | |
| Population Density | 363.6 | 3,821.0 | 93.4 |
| Land Area | 1.7 | 5.8 | 3,531,905.4 |
| Water Area | 0.0 | 0.1 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Mount Sidney, VA | Brambleton, VA | United States |
| Age 0 to 4 | 7.3% | 7.0% | 5.9% |
| Age 5 to 9 | 13.6% | 10.4% | 6.1% |
| Age 10 to 14 | 5.3% | 11.6% | 6.6% |
| Age 15 to 17 | 1.8% | 4.6% | 3.9% |
| Age 18 to 20 | 1.0% | 2.0% | 4.0% |
| Age 21 to 24 | 0.0% | 2.1% | 5.2% |
| Age 25 to 34 | 25.8% | 10.2% | 13.8% |
| Age 35 to 44 | 19.4% | 23.4% | 12.9% |
| Age 45 to 54 | 4.1% | 17.2% | 12.6% |
| Age 55 to 59 | 9.6% | 3.6% | 6.7% |
| Age 60 to 64 | 7.6% | 2.9% | 6.3% |
| Age 65 to 74 | 4.5% | 4.0% | 9.6% |
| Age 75 to 84 | 0.0% | 0.6% | 4.5% |
| Age 85 and over | 0.0% | 0.4% | 1.9% |
| space1 | | | |
| Race | | | |
| | Mount Sidney, VA | Brambleton, VA | United States |
| White | 74.3% | 39.3% | 59.4% |
| Black | 0.0% | 8.5% | 12.2% |
| Asian | 2.5% | 40.2% | 5.6% |
| Native American | 0.0% | 0.3% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.1% | 0.2% |
| Other | 0.0% | 0.1% | 0.4% |
| Two or More Races | 17.9% | 4.5% | 3.2% |
| Hispanic | 5.3% | 7.1% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Mount Sidney, VA | Brambleton, VA | United States |
| Married Population | 79.4% | 73.6% | 49.8% |
| Currently Married | 79.4% | 73.5% | 48.0% |
| Married but Separated | 0.0% | 0.1% | 1.8% |
| Single Population | 20.6% | 26.4% | 50.2% |
| Never Married | 7.6% | 19.2% | 33.8% |
| Divorced | 9.6% | 5.1% | 10.8% |
| Widowed | 3.4% | 2.2% | 5.6% |
| space5 | | | |
| Household Size | 2.56 | 3.19 | 2.60 |
| Households | 236 | 6,888 | 124,010,992 |
| Family Households | 221 | 6,095 | 80,755,759 |
| Married couple, w/children | 49.3% | 66.8% | 30.5% |
| Married couple, no children | 31.2% | 23.7% | 42.9% |
| Non Family Households | 15 | 793 | 43,255,233 |
| Single householder, w/children | 19.5% | 5.9% | 15.8% |
| Single householder, no children | 0.0% | 3.6% | 10.8% |