| | Darien, CT | Guilford, CT | United States |
| Population | 21,485 | 22,113 | 329,725,481 |
| Female Population | 50.4% | 48.8% | 50.5% |
| Male Population | 49.6% | 51.2% | 49.5% |
| Median Age | 39.9 | 48.9 | 38.4 |
| space3 | | | |
| Population - 2010 | 20,752 | 22,382 | 309,327,143 |
| Population - 2000 | 19,607 | 21,398 | 285,036,114 |
| Population - 1990 | 18,196 | 19,848 | 251,960,433 |
| Pop. 2000 to Now | 10.6% | 3.0% | 15.6% |
| Pop. 1990 to Now | 19.2% | 11.1% | 30.8% |
| space4 | | | |
| Population Density | 1,697.9 | 469.7 | 93.4 |
| Land Area | 12.7 | 47.1 | 3,531,905.4 |
| Water Area | 10.8 | 2.6 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Darien, CT | Guilford, CT | United States |
| Age 0 to 4 | 6.4% | 5.2% | 5.9% |
| Age 5 to 9 | 9.3% | 5.0% | 6.1% |
| Age 10 to 14 | 10.4% | 6.5% | 6.6% |
| Age 15 to 17 | 5.9% | 4.6% | 3.9% |
| Age 18 to 20 | 3.4% | 2.9% | 4.0% |
| Age 21 to 24 | 3.3% | 3.3% | 5.2% |
| Age 25 to 34 | 5.3% | 6.8% | 13.8% |
| Age 35 to 44 | 13.8% | 11.3% | 12.9% |
| Age 45 to 54 | 16.4% | 13.5% | 12.6% |
| Age 55 to 59 | 6.2% | 7.8% | 6.7% |
| Age 60 to 64 | 6.4% | 8.3% | 6.3% |
| Age 65 to 74 | 7.7% | 14.5% | 9.6% |
| Age 75 to 84 | 3.1% | 7.6% | 4.5% |
| Age 85 and over | 2.5% | 2.7% | 1.9% |
| space1 | | | |
| Race | | | |
| | Darien, CT | Guilford, CT | United States |
| White | 84.8% | 86.7% | 59.4% |
| Black | 0.8% | 2.2% | 12.2% |
| Asian | 6.1% | 4.2% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.3% | 0.1% | 0.4% |
| Two or More Races | 3.1% | 1.6% | 3.2% |
| Hispanic | 4.9% | 5.2% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Darien, CT | Guilford, CT | United States |
| Married Population | 67.1% | 62.7% | 49.8% |
| Currently Married | 65.3% | 62.2% | 48.0% |
| Married but Separated | 1.8% | 0.5% | 1.8% |
| Single Population | 32.9% | 37.3% | 50.2% |
| Never Married | 24.4% | 22.9% | 33.8% |
| Divorced | 4.3% | 8.7% | 10.8% |
| Widowed | 4.2% | 5.7% | 5.6% |
| space5 | | | |
| Household Size | 3.07 | 2.52 | 2.60 |
| Households | 6,978 | 8,665 | 124,010,992 |
| Family Households | 5,743 | 6,254 | 80,755,759 |
| Married couple, w/children | 50.4% | 34.7% | 30.5% |
| Married couple, no children | 38.0% | 53.4% | 42.9% |
| Non Family Households | 1,235 | 2,411 | 43,255,233 |
| Single householder, w/children | 4.7% | 5.5% | 15.8% |
| Single householder, no children | 6.8% | 6.5% | 10.8% |