| | Walnut, CA | Brea, CA | United States |
| Population | 28,641 | 46,912 | 329,725,481 |
| Female Population | 51.4% | 52.9% | 50.5% |
| Male Population | 48.6% | 47.1% | 49.5% |
| Median Age | 45.8 | 40.3 | 38.4 |
| space3 | | | |
| Population - 2010 | 29,222 | 39,449 | 309,327,143 |
| Population - 2000 | 30,004 | 35,410 | 285,036,114 |
| Population - 1990 | 29,105 | 32,873 | 251,960,433 |
| Pop. 2000 to Now | -2.3% | 25.6% | 15.6% |
| Pop. 1990 to Now | 0.8% | 35.3% | 30.8% |
| space4 | | | |
| Population Density | 3,185.2 | 3,850.0 | 93.4 |
| Land Area | 9.0 | 12.2 | 3,531,905.4 |
| Water Area | 0.0 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Walnut, CA | Brea, CA | United States |
| Age 0 to 4 | 4.2% | 5.4% | 5.9% |
| Age 5 to 9 | 5.3% | 6.2% | 6.1% |
| Age 10 to 14 | 6.4% | 6.3% | 6.6% |
| Age 15 to 17 | 3.4% | 4.2% | 3.9% |
| Age 18 to 20 | 2.3% | 3.5% | 4.0% |
| Age 21 to 24 | 4.3% | 4.6% | 5.2% |
| Age 25 to 34 | 12.2% | 13.2% | 13.8% |
| Age 35 to 44 | 11.1% | 13.0% | 12.9% |
| Age 45 to 54 | 12.5% | 15.1% | 12.6% |
| Age 55 to 59 | 8.6% | 7.3% | 6.7% |
| Age 60 to 64 | 8.6% | 5.8% | 6.3% |
| Age 65 to 74 | 14.0% | 9.5% | 9.6% |
| Age 75 to 84 | 5.5% | 3.9% | 4.5% |
| Age 85 and over | 1.6% | 2.1% | 1.9% |
| space1 | | | |
| Race | | | |
| | Walnut, CA | Brea, CA | United States |
| White | 9.8% | 39.6% | 59.4% |
| Black | 3.1% | 1.1% | 12.2% |
| Asian | 65.2% | 25.2% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.6% | 0.2% | 0.2% |
| Other | 0.3% | 0.4% | 0.4% |
| Two or More Races | 2.6% | 3.8% | 3.2% |
| Hispanic | 18.5% | 29.8% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Walnut, CA | Brea, CA | United States |
| Married Population | 60.2% | 55.5% | 49.8% |
| Currently Married | 59.0% | 54.1% | 48.0% |
| Married but Separated | 1.2% | 1.5% | 1.8% |
| Single Population | 39.8% | 44.5% | 50.2% |
| Never Married | 29.5% | 31.7% | 33.8% |
| Divorced | 5.4% | 8.1% | 10.8% |
| Widowed | 4.9% | 4.7% | 5.6% |
| space5 | | | |
| Household Size | 3.30 | 2.82 | 2.60 |
| Households | 8,663 | 16,584 | 124,010,992 |
| Family Households | 7,546 | 12,322 | 80,755,759 |
| Married couple, w/children | 27.8% | 36.6% | 30.5% |
| Married couple, no children | 51.7% | 41.0% | 42.9% |
| Non Family Households | 1,117 | 4,262 | 43,255,233 |
| Single householder, w/children | 8.9% | 10.5% | 15.8% |
| Single householder, no children | 11.6% | 11.9% | 10.8% |