| | Louisville, CO | Boulder, CO | United States |
| Population | 21,091 | 104,930 | 329,725,481 |
| Female Population | 49.4% | 48.5% | 50.5% |
| Male Population | 50.6% | 51.5% | 49.5% |
| Median Age | 42.5 | 28.9 | 38.4 |
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| Population - 2010 | 18,412 | 97,724 | 309,327,143 |
| Population - 2000 | 18,937 | 94,673 | 285,036,114 |
| Population - 1990 | 12,361 | 83,312 | 251,960,433 |
| Pop. 2000 to Now | 0.7% | 13.7% | 15.6% |
| Pop. 1990 to Now | 54.2% | 29.2% | 30.8% |
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| Population Density | 2,644.0 | 3,991.4 | 93.4 |
| Land Area | 8.0 | 26.3 | 3,531,905.4 |
| Water Area | 0.1 | 1.0 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Louisville, CO | Boulder, CO | United States |
| Age 0 to 4 | 4.5% | 2.7% | 5.9% |
| Age 5 to 9 | 6.8% | 3.7% | 6.1% |
| Age 10 to 14 | 8.0% | 3.6% | 6.6% |
| Age 15 to 17 | 4.3% | 2.5% | 3.9% |
| Age 18 to 20 | 2.8% | 16.2% | 4.0% |
| Age 21 to 24 | 3.4% | 13.8% | 5.2% |
| Age 25 to 34 | 11.2% | 16.3% | 13.8% |
| Age 35 to 44 | 13.0% | 10.3% | 12.9% |
| Age 45 to 54 | 15.1% | 10.3% | 12.6% |
| Age 55 to 59 | 8.6% | 4.5% | 6.7% |
| Age 60 to 64 | 8.0% | 4.2% | 6.3% |
| Age 65 to 74 | 9.0% | 7.0% | 9.6% |
| Age 75 to 84 | 3.0% | 3.1% | 4.5% |
| Age 85 and over | 2.3% | 1.8% | 1.9% |
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| Race | | | |
| | Louisville, CO | Boulder, CO | United States |
| White | 82.3% | 78.3% | 59.4% |
| Black | 0.6% | 1.0% | 12.2% |
| Asian | 4.1% | 5.9% | 5.6% |
| Native American | 0.0% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.1% | 0.3% | 0.4% |
| Two or More Races | 4.0% | 3.5% | 3.2% |
| Hispanic | 8.8% | 10.6% | 18.4% |
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| Marriage & Family | | | |
| | Louisville, CO | Boulder, CO | United States |
| Married Population | 56.9% | 31.9% | 49.8% |
| Currently Married | 56.6% | 31.2% | 48.0% |
| Married but Separated | 0.3% | 0.7% | 1.8% |
| Single Population | 43.1% | 68.1% | 50.2% |
| Never Married | 27.8% | 56.1% | 33.8% |
| Divorced | 10.9% | 9.5% | 10.8% |
| Widowed | 4.4% | 2.5% | 5.6% |
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| Household Size | 2.50 | 2.21 | 2.60 |
| Households | 8,400 | 42,610 | 124,010,992 |
| Family Households | 5,519 | 17,341 | 80,755,759 |
| Married couple, w/children | 38.7% | 33.3% | 30.5% |
| Married couple, no children | 43.2% | 47.4% | 42.9% |
| Non Family Households | 2,881 | 25,269 | 43,255,233 |
| Single householder, w/children | 12.2% | 10.8% | 15.8% |
| Single householder, no children | 5.9% | 8.5% | 10.8% |