| | Johnston, IA | Cumming, GA | United States |
| Population | 23,418 | 6,886 | 329,725,481 |
| Female Population | 50.8% | 51.2% | 50.5% |
| Male Population | 49.2% | 48.8% | 49.5% |
| Median Age | 37.7 | 40 | 38.4 |
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| Population - 2010 | 17,388 | 5,518 | 309,327,143 |
| Population - 2000 | 8,649 | 4,220 | 285,036,114 |
| Population - 1990 | 4,702 | 2,828 | 251,960,433 |
| Pop. 2000 to Now | 167.0% | 57.3% | 15.6% |
| Pop. 1990 to Now | 391.1% | 134.8% | 30.8% |
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| Population Density | 1,397.7 | 1,029.5 | 93.4 |
| Land Area | 16.8 | 6.7 | 3,531,905.4 |
| Water Area | 1.5 | 0.0 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Johnston, IA | Cumming, GA | United States |
| Age 0 to 4 | 7.1% | 6.5% | 5.9% |
| Age 5 to 9 | 8.6% | 2.1% | 6.1% |
| Age 10 to 14 | 8.1% | 8.0% | 6.6% |
| Age 15 to 17 | 5.0% | 2.3% | 3.9% |
| Age 18 to 20 | 1.7% | 4.1% | 4.0% |
| Age 21 to 24 | 3.7% | 7.9% | 5.2% |
| Age 25 to 34 | 12.1% | 13.5% | 13.8% |
| Age 35 to 44 | 13.5% | 13.8% | 12.9% |
| Age 45 to 54 | 14.1% | 12.4% | 12.6% |
| Age 55 to 59 | 7.0% | 6.2% | 6.7% |
| Age 60 to 64 | 4.3% | 4.5% | 6.3% |
| Age 65 to 74 | 9.1% | 7.5% | 9.6% |
| Age 75 to 84 | 3.9% | 9.1% | 4.5% |
| Age 85 and over | 2.0% | 2.0% | 1.9% |
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| Race | | | |
| | Johnston, IA | Cumming, GA | United States |
| White | 78.3% | 62.6% | 59.4% |
| Black | 9.9% | 6.9% | 12.2% |
| Asian | 6.5% | 8.3% | 5.6% |
| Native American | 0.1% | 0.7% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.0% | 0.4% |
| Two or More Races | 2.1% | 3.0% | 3.2% |
| Hispanic | 3.2% | 18.5% | 18.4% |
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| Marriage & Family | | | |
| | Johnston, IA | Cumming, GA | United States |
| Married Population | 60.7% | 43.4% | 49.8% |
| Currently Married | 59.4% | 39.5% | 48.0% |
| Married but Separated | 1.2% | 3.9% | 1.8% |
| Single Population | 39.3% | 56.6% | 50.2% |
| Never Married | 25.1% | 31.4% | 33.8% |
| Divorced | 9.3% | 16.6% | 10.8% |
| Widowed | 4.9% | 8.6% | 5.6% |
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| Household Size | 2.50 | 2.55 | 2.60 |
| Households | 9,256 | 2,503 | 124,010,992 |
| Family Households | 6,298 | 1,587 | 80,755,759 |
| Married couple, w/children | 37.4% | 20.5% | 30.5% |
| Married couple, no children | 44.1% | 42.8% | 42.9% |
| Non Family Households | 2,958 | 916 | 43,255,233 |
| Single householder, w/children | 16.4% | 20.8% | 15.8% |
| Single householder, no children | 2.1% | 15.9% | 10.8% |