| | El Cajon, CA | Billings, MT | United States |
| Population | 106,043 | 115,689 | 329,725,481 |
| Female Population | 50.8% | 50.4% | 50.5% |
| Male Population | 49.3% | 49.6% | 49.5% |
| Median Age | 34.8 | 37.6 | 38.4 |
| space3 | | | |
| Population - 2010 | 99,815 | 104,524 | 309,327,143 |
| Population - 2000 | 94,869 | 89,847 | 285,036,114 |
| Population - 1990 | 88,693 | 81,151 | 251,960,433 |
| Pop. 2000 to Now | 7.9% | 22.1% | 15.6% |
| Pop. 1990 to Now | 15.5% | 35.2% | 30.8% |
| space4 | | | |
| Population Density | 7,320.9 | 2,630.3 | 93.4 |
| Land Area | 14.5 | 44.0 | 3,531,905.4 |
| Water Area | 0.0 | 0.1 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | El Cajon, CA | Billings, MT | United States |
| Age 0 to 4 | 7.2% | 6.2% | 5.9% |
| Age 5 to 9 | 6.6% | 6.3% | 6.1% |
| Age 10 to 14 | 7.4% | 6.7% | 6.6% |
| Age 15 to 17 | 4.2% | 3.8% | 3.9% |
| Age 18 to 20 | 3.9% | 3.3% | 4.0% |
| Age 21 to 24 | 5.3% | 5.4% | 5.2% |
| Age 25 to 34 | 15.7% | 14.7% | 13.8% |
| Age 35 to 44 | 14.0% | 13.2% | 12.9% |
| Age 45 to 54 | 10.9% | 10.6% | 12.6% |
| Age 55 to 59 | 5.8% | 5.7% | 6.7% |
| Age 60 to 64 | 6.2% | 6.7% | 6.3% |
| Age 65 to 74 | 7.9% | 10.0% | 9.6% |
| Age 75 to 84 | 3.4% | 5.0% | 4.5% |
| Age 85 and over | 1.6% | 2.4% | 1.9% |
| space1 | | | |
| Race | | | |
| | El Cajon, CA | Billings, MT | United States |
| White | 56.7% | 83.7% | 59.4% |
| Black | 6.2% | 0.5% | 12.2% |
| Asian | 4.3% | 0.8% | 5.6% |
| Native American | 0.1% | 4.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.5% | 0.0% | 0.2% |
| Other | 0.4% | 0.2% | 0.4% |
| Two or More Races | 4.7% | 3.6% | 3.2% |
| Hispanic | 27.1% | 6.9% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | El Cajon, CA | Billings, MT | United States |
| Married Population | 46.3% | 48.3% | 49.8% |
| Currently Married | 43.9% | 47.5% | 48.0% |
| Married but Separated | 2.4% | 0.8% | 1.8% |
| Single Population | 53.7% | 51.7% | 50.2% |
| Never Married | 37.6% | 31.3% | 33.8% |
| Divorced | 10.5% | 14.7% | 10.8% |
| Widowed | 5.7% | 5.6% | 5.6% |
| space5 | | | |
| Household Size | 2.96 | 2.33 | 2.60 |
| Households | 35,003 | 48,478 | 124,010,992 |
| Family Households | 25,018 | 28,249 | 80,755,759 |
| Married couple, w/children | 34.1% | 31.5% | 30.5% |
| Married couple, no children | 30.9% | 43.8% | 42.9% |
| Non Family Households | 9,985 | 20,229 | 43,255,233 |
| Single householder, w/children | 20.4% | 17.1% | 15.8% |
| Single householder, no children | 14.6% | 7.6% | 10.8% |