| | San Mateo, CA | San Marcos, CA | United States |
| Population | 105,087 | 94,313 | 329,725,481 |
| Female Population | 50.4% | 50.1% | 50.5% |
| Male Population | 49.6% | 49.9% | 49.5% |
| Median Age | 37.7 | 35.6 | 38.4 |
| space3 | | | |
| Population - 2010 | 97,256 | 83,948 | 309,327,143 |
| Population - 2000 | 92,482 | 54,977 | 285,036,114 |
| Population - 1990 | 85,486 | 38,974 | 251,960,433 |
| Pop. 2000 to Now | 10.7% | 77.3% | 15.6% |
| Pop. 1990 to Now | 19.8% | 150.1% | 30.8% |
| space4 | | | |
| Population Density | 8,632.8 | 3,872.3 | 93.4 |
| Land Area | 12.2 | 24.4 | 3,531,905.4 |
| Water Area | 3.7 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | San Mateo, CA | San Marcos, CA | United States |
| Age 0 to 4 | 6.7% | 6.9% | 5.9% |
| Age 5 to 9 | 5.6% | 8.0% | 6.1% |
| Age 10 to 14 | 5.7% | 8.0% | 6.6% |
| Age 15 to 17 | 2.7% | 3.5% | 3.9% |
| Age 18 to 20 | 2.6% | 5.2% | 4.0% |
| Age 21 to 24 | 4.1% | 4.5% | 5.2% |
| Age 25 to 34 | 17.5% | 12.9% | 13.8% |
| Age 35 to 44 | 15.4% | 15.9% | 12.9% |
| Age 45 to 54 | 12.7% | 12.4% | 12.6% |
| Age 55 to 59 | 5.8% | 5.5% | 6.7% |
| Age 60 to 64 | 5.7% | 4.6% | 6.3% |
| Age 65 to 74 | 8.0% | 7.5% | 9.6% |
| Age 75 to 84 | 4.8% | 3.3% | 4.5% |
| Age 85 and over | 2.8% | 1.8% | 1.9% |
| space1 | | | |
| Race | | | |
| | San Mateo, CA | San Marcos, CA | United States |
| White | 40.9% | 43.5% | 59.4% |
| Black | 1.9% | 1.5% | 12.2% |
| Asian | 25.2% | 10.5% | 5.6% |
| Native American | 0.1% | 0.3% | 0.6% |
| Hawaiian, Pacific Islander | 1.7% | 0.0% | 0.2% |
| Other | 0.6% | 0.2% | 0.4% |
| Two or More Races | 5.5% | 4.2% | 3.2% |
| Hispanic | 24.1% | 39.7% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | San Mateo, CA | San Marcos, CA | United States |
| Married Population | 52.6% | 54.1% | 49.8% |
| Currently Married | 51.3% | 52.7% | 48.0% |
| Married but Separated | 1.2% | 1.4% | 1.8% |
| Single Population | 47.4% | 45.9% | 50.2% |
| Never Married | 33.0% | 31.5% | 33.8% |
| Divorced | 8.9% | 9.1% | 10.8% |
| Widowed | 5.6% | 5.2% | 5.6% |
| space5 | | | |
| Household Size | 2.58 | 3.08 | 2.60 |
| Households | 40,233 | 30,304 | 124,010,992 |
| Family Households | 25,420 | 22,517 | 80,755,759 |
| Married couple, w/children | 35.1% | 40.9% | 30.5% |
| Married couple, no children | 43.3% | 35.3% | 42.9% |
| Non Family Households | 14,813 | 7,787 | 43,255,233 |
| Single householder, w/children | 10.4% | 14.4% | 15.8% |
| Single householder, no children | 11.3% | 9.4% | 10.8% |