| | South San Francisco, CA | Seattle, WA | United States |
| Population | 66,331 | 726,054 | 329,725,481 |
| Female Population | 49.8% | 49.3% | 50.5% |
| Male Population | 50.2% | 50.7% | 49.5% |
| Median Age | 42.1 | 35.3 | 38.4 |
| space3 | | | |
| Population - 2010 | 63,674 | 610,654 | 309,327,143 |
| Population - 2000 | 60,552 | 563,374 | 285,036,114 |
| Population - 1990 | 54,312 | 516,259 | 251,960,433 |
| Pop. 2000 to Now | 9.9% | 36.6% | 15.6% |
| Pop. 1990 to Now | 22.6% | 49.1% | 30.8% |
| space4 | | | |
| Population Density | 7,189.6 | 8,657.8 | 93.4 |
| Land Area | 9.2 | 83.9 | 3,531,905.4 |
| Water Area | 21.0 | 58.2 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | South San Francisco, CA | Seattle, WA | United States |
| Age 0 to 4 | 4.6% | 4.6% | 5.9% |
| Age 5 to 9 | 5.4% | 4.0% | 6.1% |
| Age 10 to 14 | 4.9% | 3.9% | 6.6% |
| Age 15 to 17 | 2.8% | 2.0% | 3.9% |
| Age 18 to 20 | 3.1% | 3.8% | 4.0% |
| Age 21 to 24 | 4.4% | 6.3% | 5.2% |
| Age 25 to 34 | 14.4% | 24.8% | 13.8% |
| Age 35 to 44 | 14.1% | 15.8% | 12.9% |
| Age 45 to 54 | 14.8% | 12.3% | 12.6% |
| Age 55 to 59 | 7.6% | 5.4% | 6.7% |
| Age 60 to 64 | 7.2% | 4.8% | 6.3% |
| Age 65 to 74 | 10.1% | 7.7% | 9.6% |
| Age 75 to 84 | 4.4% | 3.1% | 4.5% |
| Age 85 and over | 2.1% | 1.5% | 1.9% |
| space1 | | | |
| Race | | | |
| | South San Francisco, CA | Seattle, WA | United States |
| White | 21.5% | 62.2% | 59.4% |
| Black | 1.8% | 6.7% | 12.2% |
| Asian | 42.8% | 16.2% | 5.6% |
| Native American | 0.2% | 0.4% | 0.6% |
| Hawaiian, Pacific Islander | 0.9% | 0.2% | 0.2% |
| Other | 0.2% | 0.4% | 0.4% |
| Two or More Races | 2.9% | 6.8% | 3.2% |
| Hispanic | 29.8% | 7.2% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | South San Francisco, CA | Seattle, WA | United States |
| Married Population | 53.3% | 41.5% | 49.8% |
| Currently Married | 51.9% | 40.3% | 48.0% |
| Married but Separated | 1.4% | 1.2% | 1.8% |
| Single Population | 46.7% | 58.5% | 50.2% |
| Never Married | 33.3% | 45.5% | 33.8% |
| Divorced | 8.0% | 10.0% | 10.8% |
| Widowed | 5.3% | 3.0% | 5.6% |
| space5 | | | |
| Household Size | 3.00 | 2.08 | 2.60 |
| Households | 21,895 | 337,361 | 124,010,992 |
| Family Households | 16,064 | 148,959 | 80,755,759 |
| Married couple, w/children | 31.7% | 31.7% | 30.5% |
| Married couple, no children | 43.8% | 48.3% | 42.9% |
| Non Family Households | 5,831 | 188,402 | 43,255,233 |
| Single householder, w/children | 8.9% | 10.7% | 15.8% |
| Single householder, no children | 15.6% | 9.3% | 10.8% |