| | San Diego, TX | Brenham, TX | United States |
| Population | 3,705 | 17,385 | 329,725,481 |
| Female Population | 46.8% | 54.6% | 50.5% |
| Male Population | 53.2% | 45.4% | 49.5% |
| Median Age | 36.9 | 35 | 38.4 |
| space3 | | | |
| Population - 2010 | 4,450 | 15,835 | 309,327,143 |
| Population - 2000 | 4,753 | 13,507 | 285,036,114 |
| Population - 1990 | 4,983 | 11,952 | 251,960,433 |
| Pop. 2000 to Now | -11.9% | 31.3% | 15.6% |
| Pop. 1990 to Now | -16.0% | 48.4% | 30.8% |
| space4 | | | |
| Population Density | 1,949.0 | 1,342.0 | 93.4 |
| Land Area | 1.9 | 13.0 | 3,531,905.4 |
| Water Area | 0.0 | 0.1 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | San Diego, TX | Brenham, TX | United States |
| Age 0 to 4 | 5.5% | 5.4% | 5.9% |
| Age 5 to 9 | 6.4% | 6.8% | 6.1% |
| Age 10 to 14 | 13.0% | 6.6% | 6.6% |
| Age 15 to 17 | 4.0% | 4.0% | 3.9% |
| Age 18 to 20 | 1.8% | 10.9% | 4.0% |
| Age 21 to 24 | 7.9% | 6.7% | 5.2% |
| Age 25 to 34 | 5.2% | 9.6% | 13.8% |
| Age 35 to 44 | 15.5% | 11.1% | 12.9% |
| Age 45 to 54 | 12.9% | 9.2% | 12.6% |
| Age 55 to 59 | 8.5% | 5.4% | 6.7% |
| Age 60 to 64 | 4.5% | 5.9% | 6.3% |
| Age 65 to 74 | 9.2% | 8.0% | 9.6% |
| Age 75 to 84 | 4.1% | 7.2% | 4.5% |
| Age 85 and over | 1.6% | 3.2% | 1.9% |
| space1 | | | |
| Race | | | |
| | San Diego, TX | Brenham, TX | United States |
| White | 1.7% | 47.7% | 59.4% |
| Black | 0.0% | 22.6% | 12.2% |
| Asian | 0.0% | 1.7% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.3% | 0.4% |
| Two or More Races | 0.0% | 2.6% | 3.2% |
| Hispanic | 98.3% | 25.1% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | San Diego, TX | Brenham, TX | United States |
| Married Population | 36.2% | 43.5% | 49.8% |
| Currently Married | 34.2% | 42.4% | 48.0% |
| Married but Separated | 1.9% | 1.1% | 1.8% |
| Single Population | 63.8% | 56.5% | 50.2% |
| Never Married | 42.2% | 37.8% | 33.8% |
| Divorced | 14.3% | 12.8% | 10.8% |
| Widowed | 7.3% | 5.9% | 5.6% |
| space5 | | | |
| Household Size | 3.17 | 2.36 | 2.60 |
| Households | 1,153 | 6,476 | 124,010,992 |
| Family Households | 668 | 3,755 | 80,755,759 |
| Married couple, w/children | 10.8% | 31.4% | 30.5% |
| Married couple, no children | 40.0% | 40.4% | 42.9% |
| Non Family Households | 485 | 2,721 | 43,255,233 |
| Single householder, w/children | 29.2% | 19.0% | 15.8% |
| Single householder, no children | 20.1% | 9.2% | 10.8% |