| | San Marcos, CA | Simi Valley, CA | United States |
| Population | 94,313 | 126,809 | 329,725,481 |
| Female Population | 50.1% | 51.0% | 50.5% |
| Male Population | 49.9% | 49.1% | 49.5% |
| Median Age | 35.6 | 41 | 38.4 |
| space3 | | | |
| Population - 2010 | 83,948 | 124,424 | 309,327,143 |
| Population - 2000 | 54,977 | 111,351 | 285,036,114 |
| Population - 1990 | 38,974 | 100,217 | 251,960,433 |
| Pop. 2000 to Now | 77.3% | 12.4% | 15.6% |
| Pop. 1990 to Now | 150.1% | 24.9% | 30.8% |
| space4 | | | |
| Population Density | 3,872.3 | 3,054.0 | 93.4 |
| Land Area | 24.4 | 41.5 | 3,531,905.4 |
| Water Area | 0.0 | 0.7 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | San Marcos, CA | Simi Valley, CA | United States |
| Age 0 to 4 | 6.9% | 5.2% | 5.9% |
| Age 5 to 9 | 8.0% | 4.8% | 6.1% |
| Age 10 to 14 | 8.0% | 6.6% | 6.6% |
| Age 15 to 17 | 3.5% | 4.2% | 3.9% |
| Age 18 to 20 | 5.2% | 3.8% | 4.0% |
| Age 21 to 24 | 4.5% | 4.6% | 5.2% |
| Age 25 to 34 | 12.9% | 13.3% | 13.8% |
| Age 35 to 44 | 15.9% | 12.5% | 12.9% |
| Age 45 to 54 | 12.4% | 14.8% | 12.6% |
| Age 55 to 59 | 5.5% | 7.7% | 6.7% |
| Age 60 to 64 | 4.6% | 6.7% | 6.3% |
| Age 65 to 74 | 7.5% | 9.2% | 9.6% |
| Age 75 to 84 | 3.3% | 4.5% | 4.5% |
| Age 85 and over | 1.8% | 2.0% | 1.9% |
| space1 | | | |
| Race | | | |
| | San Marcos, CA | Simi Valley, CA | United States |
| White | 43.5% | 57.6% | 59.4% |
| Black | 1.5% | 1.3% | 12.2% |
| Asian | 10.5% | 11.1% | 5.6% |
| Native American | 0.3% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.1% | 0.2% |
| Other | 0.2% | 0.1% | 0.4% |
| Two or More Races | 4.2% | 3.8% | 3.2% |
| Hispanic | 39.7% | 25.8% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | San Marcos, CA | Simi Valley, CA | United States |
| Married Population | 54.1% | 50.5% | 49.8% |
| Currently Married | 52.7% | 49.1% | 48.0% |
| Married but Separated | 1.4% | 1.4% | 1.8% |
| Single Population | 45.9% | 49.5% | 50.2% |
| Never Married | 31.5% | 33.6% | 33.8% |
| Divorced | 9.1% | 10.1% | 10.8% |
| Widowed | 5.2% | 5.9% | 5.6% |
| space5 | | | |
| Household Size | 3.08 | 2.87 | 2.60 |
| Households | 30,304 | 43,893 | 124,010,992 |
| Family Households | 22,517 | 30,965 | 80,755,759 |
| Married couple, w/children | 40.9% | 32.3% | 30.5% |
| Married couple, no children | 35.3% | 43.9% | 42.9% |
| Non Family Households | 7,787 | 12,928 | 43,255,233 |
| Single householder, w/children | 14.4% | 10.9% | 15.8% |
| Single householder, no children | 9.4% | 12.8% | 10.8% |