| | San Carlos, CA | Petaluma, CA | United States |
| Population | 30,754 | 60,050 | 329,725,481 |
| Female Population | 51.1% | 50.1% | 50.5% |
| Male Population | 48.9% | 49.9% | 49.5% |
| Median Age | 41 | 42.8 | 38.4 |
| space3 | | | |
| Population - 2010 | 28,300 | 58,048 | 309,327,143 |
| Population - 2000 | 27,718 | 54,548 | 285,036,114 |
| Population - 1990 | 26,167 | 43,184 | 251,960,433 |
| Pop. 2000 to Now | 6.9% | 11.2% | 15.6% |
| Pop. 1990 to Now | 13.3% | 40.4% | 30.8% |
| space4 | | | |
| Population Density | 5,578.5 | 4,175.4 | 93.4 |
| Land Area | 5.5 | 14.4 | 3,531,905.4 |
| Water Area | 0.0 | 0.1 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | San Carlos, CA | Petaluma, CA | United States |
| Age 0 to 4 | 5.4% | 5.0% | 5.9% |
| Age 5 to 9 | 7.2% | 5.5% | 6.1% |
| Age 10 to 14 | 7.0% | 6.1% | 6.6% |
| Age 15 to 17 | 4.0% | 4.0% | 3.9% |
| Age 18 to 20 | 1.3% | 3.1% | 4.0% |
| Age 21 to 24 | 3.8% | 3.0% | 5.2% |
| Age 25 to 34 | 10.7% | 12.7% | 13.8% |
| Age 35 to 44 | 15.6% | 13.7% | 12.9% |
| Age 45 to 54 | 15.1% | 13.5% | 12.6% |
| Age 55 to 59 | 8.8% | 7.4% | 6.7% |
| Age 60 to 64 | 5.8% | 7.3% | 6.3% |
| Age 65 to 74 | 8.7% | 11.1% | 9.6% |
| Age 75 to 84 | 4.4% | 5.0% | 4.5% |
| Age 85 and over | 2.1% | 2.6% | 1.9% |
| space1 | | | |
| Race | | | |
| | San Carlos, CA | Petaluma, CA | United States |
| White | 63.2% | 68.1% | 59.4% |
| Black | 0.7% | 1.3% | 12.2% |
| Asian | 18.2% | 4.1% | 5.6% |
| Native American | 0.1% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.4% | 0.0% | 0.2% |
| Other | 1.0% | 0.2% | 0.4% |
| Two or More Races | 6.9% | 4.4% | 3.2% |
| Hispanic | 9.5% | 21.6% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | San Carlos, CA | Petaluma, CA | United States |
| Married Population | 61.0% | 53.4% | 49.8% |
| Currently Married | 60.0% | 51.8% | 48.0% |
| Married but Separated | 1.0% | 1.5% | 1.8% |
| Single Population | 39.0% | 46.6% | 50.2% |
| Never Married | 25.3% | 30.0% | 33.8% |
| Divorced | 9.2% | 11.6% | 10.8% |
| Widowed | 4.6% | 5.0% | 5.6% |
| space5 | | | |
| Household Size | 2.69 | 2.61 | 2.60 |
| Households | 11,393 | 22,756 | 124,010,992 |
| Family Households | 8,214 | 15,059 | 80,755,759 |
| Married couple, w/children | 43.2% | 34.8% | 30.5% |
| Married couple, no children | 42.9% | 45.5% | 42.9% |
| Non Family Households | 3,179 | 7,697 | 43,255,233 |
| Single householder, w/children | 7.6% | 11.6% | 15.8% |
| Single householder, no children | 6.3% | 8.1% | 10.8% |