| | Newcastle, CA | Kelseyville, CA | United States |
| Population | 1,505 | 3,873 | 329,725,481 |
| Female Population | 50.9% | 45.8% | 50.5% |
| Male Population | 49.1% | 54.2% | 49.5% |
| Median Age | 40.9 | 29.6 | 38.4 |
| space3 | | | |
| Population - 2010 | 1,116 | 3,126 | 309,327,143 |
| Population - 2000 | 1,145 | 2,928 | 285,036,114 |
| Population - 1990 | 1,048 | 2,861 | 251,960,433 |
| Pop. 2000 to Now | 19.0% | 25.0% | 15.6% |
| Pop. 1990 to Now | 30.1% | 27.9% | 30.8% |
| space4 | | | |
| Population Density | 629.2 | 1,342.5 | 93.4 |
| Land Area | 2.4 | 2.9 | 3,531,905.4 |
| Water Area | 0.0 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Newcastle, CA | Kelseyville, CA | United States |
| Age 0 to 4 | 2.6% | 15.7% | 5.9% |
| Age 5 to 9 | 5.8% | 9.6% | 6.1% |
| Age 10 to 14 | 8.9% | 7.0% | 6.6% |
| Age 15 to 17 | 1.7% | 1.5% | 3.9% |
| Age 18 to 20 | 0.5% | 0.8% | 4.0% |
| Age 21 to 24 | 1.2% | 4.3% | 5.2% |
| Age 25 to 34 | 17.9% | 17.9% | 13.8% |
| Age 35 to 44 | 17.2% | 10.0% | 12.9% |
| Age 45 to 54 | 4.5% | 6.0% | 12.6% |
| Age 55 to 59 | 3.5% | 5.1% | 6.7% |
| Age 60 to 64 | 7.6% | 3.4% | 6.3% |
| Age 65 to 74 | 18.9% | 9.6% | 9.6% |
| Age 75 to 84 | 7.0% | 7.0% | 4.5% |
| Age 85 and over | 2.7% | 1.9% | 1.9% |
| space1 | | | |
| Race | | | |
| | Newcastle, CA | Kelseyville, CA | United States |
| White | 73.4% | 33.1% | 59.4% |
| Black | 0.0% | 1.6% | 12.2% |
| Asian | 0.7% | 0.0% | 5.6% |
| Native American | 0.3% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.0% | 0.4% |
| Two or More Races | 8.6% | 0.0% | 3.2% |
| Hispanic | 17.1% | 65.3% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Newcastle, CA | Kelseyville, CA | United States |
| Married Population | 44.7% | 49.4% | 49.8% |
| Currently Married | 44.6% | 44.7% | 48.0% |
| Married but Separated | 0.1% | 4.8% | 1.8% |
| Single Population | 55.3% | 50.6% | 50.2% |
| Never Married | 29.7% | 31.0% | 33.8% |
| Divorced | 16.2% | 14.7% | 10.8% |
| Widowed | 9.4% | 4.9% | 5.6% |
| space5 | | | |
| Household Size | 2.36 | 2.82 | 2.60 |
| Households | 634 | 1,372 | 124,010,992 |
| Family Households | 397 | 893 | 80,755,759 |
| Married couple, w/children | 21.2% | 35.1% | 30.5% |
| Married couple, no children | 45.6% | 24.4% | 42.9% |
| Non Family Households | 237 | 479 | 43,255,233 |
| Single householder, w/children | 12.1% | 22.5% | 15.8% |
| Single householder, no children | 21.2% | 18.0% | 10.8% |