| | Johnston, IA | Cary, IL | United States |
| Population | 23,418 | 17,919 | 329,725,481 |
| Female Population | 50.8% | 50.2% | 50.5% |
| Male Population | 49.2% | 49.8% | 49.5% |
| Median Age | 37.7 | 39 | 38.4 |
| space3 | | | |
| Population - 2010 | 17,388 | 18,346 | 309,327,143 |
| Population - 2000 | 8,649 | 15,531 | 285,036,114 |
| Population - 1990 | 4,702 | 10,043 | 251,960,433 |
| Pop. 2000 to Now | 167.0% | 15.4% | 15.6% |
| Pop. 1990 to Now | 391.1% | 78.5% | 30.8% |
| space4 | | | |
| Population Density | 1,397.7 | 2,881.8 | 93.4 |
| Land Area | 16.8 | 6.2 | 3,531,905.4 |
| Water Area | 1.5 | 0.1 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Johnston, IA | Cary, IL | United States |
| Age 0 to 4 | 7.1% | 5.6% | 5.9% |
| Age 5 to 9 | 8.6% | 5.3% | 6.1% |
| Age 10 to 14 | 8.1% | 8.3% | 6.6% |
| Age 15 to 17 | 5.0% | 4.4% | 3.9% |
| Age 18 to 20 | 1.7% | 3.2% | 4.0% |
| Age 21 to 24 | 3.7% | 5.7% | 5.2% |
| Age 25 to 34 | 12.1% | 11.4% | 13.8% |
| Age 35 to 44 | 13.5% | 14.7% | 12.9% |
| Age 45 to 54 | 14.1% | 14.0% | 12.6% |
| Age 55 to 59 | 7.0% | 8.5% | 6.7% |
| Age 60 to 64 | 4.3% | 7.5% | 6.3% |
| Age 65 to 74 | 9.1% | 7.8% | 9.6% |
| Age 75 to 84 | 3.9% | 2.6% | 4.5% |
| Age 85 and over | 2.0% | 1.0% | 1.9% |
| space1 | | | |
| Race | | | |
| | Johnston, IA | Cary, IL | United States |
| White | 78.3% | 84.0% | 59.4% |
| Black | 9.9% | 0.2% | 12.2% |
| Asian | 6.5% | 1.5% | 5.6% |
| Native American | 0.1% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.6% | 0.4% |
| Two or More Races | 2.1% | 3.6% | 3.2% |
| Hispanic | 3.2% | 9.9% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Johnston, IA | Cary, IL | United States |
| Married Population | 60.7% | 57.7% | 49.8% |
| Currently Married | 59.4% | 57.2% | 48.0% |
| Married but Separated | 1.2% | 0.4% | 1.8% |
| Single Population | 39.3% | 42.3% | 50.2% |
| Never Married | 25.1% | 29.3% | 33.8% |
| Divorced | 9.3% | 10.0% | 10.8% |
| Widowed | 4.9% | 3.0% | 5.6% |
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| Household Size | 2.50 | 2.84 | 2.60 |
| Households | 9,256 | 6,305 | 124,010,992 |
| Family Households | 6,298 | 4,948 | 80,755,759 |
| Married couple, w/children | 37.4% | 32.7% | 30.5% |
| Married couple, no children | 44.1% | 47.9% | 42.9% |
| Non Family Households | 2,958 | 1,357 | 43,255,233 |
| Single householder, w/children | 16.4% | 12.9% | 15.8% |
| Single householder, no children | 2.1% | 6.6% | 10.8% |