| | Marion, IA | Fort Dodge, IA | United States |
| Population | 41,023 | 24,826 | 329,725,481 |
| Female Population | 53.4% | 48.3% | 50.5% |
| Male Population | 46.6% | 51.7% | 49.5% |
| Median Age | 39.3 | 37 | 38.4 |
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| Population - 2010 | 35,324 | 25,119 | 309,327,143 |
| Population - 2000 | 26,294 | 25,136 | 285,036,114 |
| Population - 1990 | 20,403 | 25,894 | 251,960,433 |
| Pop. 2000 to Now | 55.1% | -4.9% | 15.6% |
| Pop. 1990 to Now | 99.9% | -7.7% | 30.8% |
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| Population Density | 2,331.9 | 1,547.4 | 93.4 |
| Land Area | 17.6 | 16.0 | 3,531,905.4 |
| Water Area | 0.0 | 0.3 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Marion, IA | Fort Dodge, IA | United States |
| Age 0 to 4 | 6.3% | 5.4% | 5.9% |
| Age 5 to 9 | 7.6% | 6.1% | 6.1% |
| Age 10 to 14 | 7.0% | 5.5% | 6.6% |
| Age 15 to 17 | 4.2% | 3.9% | 3.9% |
| Age 18 to 20 | 2.0% | 6.4% | 4.0% |
| Age 21 to 24 | 3.8% | 6.8% | 5.2% |
| Age 25 to 34 | 13.0% | 12.9% | 13.8% |
| Age 35 to 44 | 15.0% | 11.6% | 12.9% |
| Age 45 to 54 | 12.9% | 10.8% | 12.6% |
| Age 55 to 59 | 6.0% | 6.4% | 6.7% |
| Age 60 to 64 | 5.7% | 5.8% | 6.3% |
| Age 65 to 74 | 9.6% | 9.7% | 9.6% |
| Age 75 to 84 | 4.9% | 5.7% | 4.5% |
| Age 85 and over | 2.0% | 2.6% | 1.9% |
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| Race | | | |
| | Marion, IA | Fort Dodge, IA | United States |
| White | 90.7% | 81.3% | 59.4% |
| Black | 1.9% | 6.1% | 12.2% |
| Asian | 1.8% | 1.9% | 5.6% |
| Native American | 0.0% | 0.3% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.3% | 0.4% | 0.4% |
| Two or More Races | 2.8% | 2.8% | 3.2% |
| Hispanic | 2.5% | 7.2% | 18.4% |
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| Marriage & Family | | | |
| | Marion, IA | Fort Dodge, IA | United States |
| Married Population | 56.1% | 38.5% | 49.8% |
| Currently Married | 54.7% | 36.6% | 48.0% |
| Married but Separated | 1.4% | 2.0% | 1.8% |
| Single Population | 43.9% | 61.5% | 50.2% |
| Never Married | 26.0% | 39.0% | 33.8% |
| Divorced | 11.8% | 15.6% | 10.8% |
| Widowed | 6.1% | 6.9% | 5.6% |
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| Household Size | 2.45 | 2.12 | 2.60 |
| Households | 16,638 | 10,437 | 124,010,992 |
| Family Households | 10,617 | 5,193 | 80,755,759 |
| Married couple, w/children | 36.0% | 22.4% | 30.5% |
| Married couple, no children | 44.5% | 45.7% | 42.9% |
| Non Family Households | 6,021 | 5,244 | 43,255,233 |
| Single householder, w/children | 14.7% | 23.5% | 15.8% |
| Single householder, no children | 4.8% | 8.4% | 10.8% |