| | New Plymouth, ID | Caldwell, ID | United States |
| Population | 1,633 | 58,872 | 329,725,481 |
| Female Population | 50.5% | 51.1% | 50.5% |
| Male Population | 49.5% | 48.9% | 49.5% |
| Median Age | 37.5 | 29.9 | 38.4 |
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| Population - 2010 | 1,506 | 46,545 | 309,327,143 |
| Population - 2000 | 1,400 | 25,967 | 285,036,114 |
| Population - 1990 | 1,313 | 18,400 | 251,960,433 |
| Pop. 2000 to Now | 14.9% | 133.7% | 15.6% |
| Pop. 1990 to Now | 22.5% | 229.8% | 30.8% |
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| Population Density | 2,373.6 | 2,650.5 | 93.4 |
| Land Area | 0.7 | 22.2 | 3,531,905.4 |
| Water Area | 0.0 | 0.1 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | New Plymouth, ID | Caldwell, ID | United States |
| Age 0 to 4 | 8.4% | 9.2% | 5.9% |
| Age 5 to 9 | 6.1% | 9.3% | 6.1% |
| Age 10 to 14 | 8.5% | 9.9% | 6.6% |
| Age 15 to 17 | 4.4% | 4.6% | 3.9% |
| Age 18 to 20 | 3.1% | 5.0% | 4.0% |
| Age 21 to 24 | 5.3% | 4.9% | 5.2% |
| Age 25 to 34 | 13.0% | 15.0% | 13.8% |
| Age 35 to 44 | 13.2% | 13.0% | 12.9% |
| Age 45 to 54 | 11.0% | 10.3% | 12.6% |
| Age 55 to 59 | 5.9% | 5.3% | 6.7% |
| Age 60 to 64 | 6.1% | 4.1% | 6.3% |
| Age 65 to 74 | 6.4% | 5.1% | 9.6% |
| Age 75 to 84 | 6.1% | 3.3% | 4.5% |
| Age 85 and over | 2.4% | 1.0% | 1.9% |
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| Race | | | |
| | New Plymouth, ID | Caldwell, ID | United States |
| White | 76.5% | 56.7% | 59.4% |
| Black | 0.0% | 0.4% | 12.2% |
| Asian | 1.1% | 0.7% | 5.6% |
| Native American | 0.0% | 0.6% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.2% | 0.2% |
| Other | 0.0% | 0.3% | 0.4% |
| Two or More Races | 4.5% | 3.7% | 3.2% |
| Hispanic | 17.9% | 37.5% | 18.4% |
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| Marriage & Family | | | |
| | New Plymouth, ID | Caldwell, ID | United States |
| Married Population | 61.1% | 51.1% | 49.8% |
| Currently Married | 57.7% | 49.4% | 48.0% |
| Married but Separated | 3.4% | 1.7% | 1.8% |
| Single Population | 38.9% | 48.9% | 50.2% |
| Never Married | 17.7% | 31.1% | 33.8% |
| Divorced | 15.1% | 12.8% | 10.8% |
| Widowed | 6.0% | 5.0% | 5.6% |
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| Household Size | 2.41 | 3.15 | 2.60 |
| Households | 677 | 18,283 | 124,010,992 |
| Family Households | 505 | 12,972 | 80,755,759 |
| Married couple, w/children | 50.1% | 45.0% | 30.5% |
| Married couple, no children | 29.7% | 26.6% | 42.9% |
| Non Family Households | 172 | 5,311 | 43,255,233 |
| Single householder, w/children | 13.1% | 18.5% | 15.8% |
| Single householder, no children | 7.1% | 9.9% | 10.8% |