| | San Marcos, TX | Plainfield, IN | United States |
| Population | 64,812 | 34,311 | 329,725,481 |
| Female Population | 52.5% | 48.0% | 50.5% |
| Male Population | 47.5% | 52.0% | 49.5% |
| Median Age | 24.9 | 38.1 | 38.4 |
| space3 | | | |
| Population - 2010 | 45,258 | 27,792 | 309,327,143 |
| Population - 2000 | 34,733 | 18,396 | 285,036,114 |
| Population - 1990 | 28,743 | 10,433 | 251,960,433 |
| Pop. 2000 to Now | 91.0% | 97.2% | 15.6% |
| Pop. 1990 to Now | 130.8% | 247.7% | 30.8% |
| space4 | | | |
| Population Density | 1,905.3 | 1,361.2 | 93.4 |
| Land Area | 34.0 | 25.2 | 3,531,905.4 |
| Water Area | 0.1 | 0.1 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | San Marcos, TX | Plainfield, IN | United States |
| Age 0 to 4 | 3.7% | 5.3% | 5.9% |
| Age 5 to 9 | 3.3% | 7.7% | 6.1% |
| Age 10 to 14 | 3.6% | 5.6% | 6.6% |
| Age 15 to 17 | 2.4% | 4.8% | 3.9% |
| Age 18 to 20 | 16.9% | 3.6% | 4.0% |
| Age 21 to 24 | 20.5% | 5.8% | 5.2% |
| Age 25 to 34 | 17.3% | 13.4% | 13.8% |
| Age 35 to 44 | 9.6% | 14.4% | 12.9% |
| Age 45 to 54 | 6.6% | 11.7% | 12.6% |
| Age 55 to 59 | 3.4% | 6.8% | 6.7% |
| Age 60 to 64 | 3.6% | 5.6% | 6.3% |
| Age 65 to 74 | 6.1% | 8.2% | 9.6% |
| Age 75 to 84 | 1.9% | 5.6% | 4.5% |
| Age 85 and over | 1.3% | 1.5% | 1.9% |
| space1 | | | |
| Race | | | |
| | San Marcos, TX | Plainfield, IN | United States |
| White | 45.2% | 78.4% | 59.4% |
| Black | 6.6% | 7.8% | 12.2% |
| Asian | 2.5% | 4.7% | 5.6% |
| Native American | 0.2% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 1.4% | 0.7% | 0.4% |
| Two or More Races | 3.1% | 2.4% | 3.2% |
| Hispanic | 40.9% | 5.8% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | San Marcos, TX | Plainfield, IN | United States |
| Married Population | 27.8% | 51.4% | 49.8% |
| Currently Married | 26.2% | 50.4% | 48.0% |
| Married but Separated | 1.6% | 1.0% | 1.8% |
| Single Population | 72.2% | 48.6% | 50.2% |
| Never Married | 61.2% | 30.0% | 33.8% |
| Divorced | 8.1% | 12.9% | 10.8% |
| Widowed | 2.8% | 5.8% | 5.6% |
| space5 | | | |
| Household Size | 2.29 | 2.57 | 2.60 |
| Households | 25,732 | 12,715 | 124,010,992 |
| Family Households | 11,052 | 8,687 | 80,755,759 |
| Married couple, w/children | 21.9% | 34.9% | 30.5% |
| Married couple, no children | 40.8% | 40.7% | 42.9% |
| Non Family Households | 14,680 | 4,028 | 43,255,233 |
| Single householder, w/children | 16.7% | 15.2% | 15.8% |
| Single householder, no children | 20.6% | 9.3% | 10.8% |