| | Shelby, IA | Denison, IA | United States |
| Population | 689 | 8,323 | 329,725,481 |
| Female Population | 48.6% | 53.0% | 50.5% |
| Male Population | 51.4% | 47.0% | 49.5% |
| Median Age | 39.1 | 33.4 | 38.4 |
| space3 | | | |
| Population - 2010 | 640 | 8,329 | 309,327,143 |
| Population - 2000 | 696 | 7,339 | 285,036,114 |
| Population - 1990 | 637 | 6,604 | 251,960,433 |
| Pop. 2000 to Now | -12.8% | 12.4% | 15.6% |
| Pop. 1990 to Now | -4.7% | 24.9% | 30.8% |
| space4 | | | |
| Population Density | 368.3 | 1,274.2 | 93.4 |
| Land Area | 1.9 | 6.5 | 3,531,905.4 |
| Water Area | 0.0 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Shelby, IA | Denison, IA | United States |
| Age 0 to 4 | 3.3% | 5.4% | 5.9% |
| Age 5 to 9 | 3.3% | 8.0% | 6.1% |
| Age 10 to 14 | 6.0% | 8.1% | 6.6% |
| Age 15 to 17 | 5.5% | 4.7% | 3.9% |
| Age 18 to 20 | 3.5% | 7.2% | 4.0% |
| Age 21 to 24 | 8.0% | 5.3% | 5.2% |
| Age 25 to 34 | 16.4% | 15.1% | 13.8% |
| Age 35 to 44 | 10.0% | 12.5% | 12.9% |
| Age 45 to 54 | 12.0% | 9.7% | 12.6% |
| Age 55 to 59 | 9.9% | 7.2% | 6.7% |
| Age 60 to 64 | 5.2% | 3.7% | 6.3% |
| Age 65 to 74 | 11.9% | 7.0% | 9.6% |
| Age 75 to 84 | 3.9% | 5.6% | 4.5% |
| Age 85 and over | 1.0% | 0.5% | 1.9% |
| space1 | | | |
| Race | | | |
| | Shelby, IA | Denison, IA | United States |
| White | 95.6% | 40.4% | 59.4% |
| Black | 0.0% | 6.4% | 12.2% |
| Asian | 0.0% | 4.3% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.0% | 0.4% |
| Two or More Races | 2.3% | 0.5% | 3.2% |
| Hispanic | 2.0% | 48.4% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Shelby, IA | Denison, IA | United States |
| Married Population | 45.0% | 46.6% | 49.8% |
| Currently Married | 44.5% | 44.1% | 48.0% |
| Married but Separated | 0.5% | 2.5% | 1.8% |
| Single Population | 55.0% | 53.4% | 50.2% |
| Never Married | 37.2% | 36.5% | 33.8% |
| Divorced | 13.6% | 9.7% | 10.8% |
| Widowed | 4.2% | 7.2% | 5.6% |
| space5 | | | |
| Household Size | 2.23 | 2.64 | 2.60 |
| Households | 303 | 2,972 | 124,010,992 |
| Family Households | 168 | 2,216 | 80,755,759 |
| Married couple, w/children | 25.0% | 23.4% | 30.5% |
| Married couple, no children | 50.6% | 35.2% | 42.9% |
| Non Family Households | 135 | 756 | 43,255,233 |
| Single householder, w/children | 8.9% | 36.9% | 15.8% |
| Single householder, no children | 15.5% | 4.5% | 10.8% |