| | Walnut, IA | Harlan, IA | United States |
| Population | 686 | 4,912 | 329,725,481 |
| Female Population | 50.2% | 53.8% | 50.5% |
| Male Population | 49.9% | 46.3% | 49.5% |
| Median Age | 51.2 | 48.1 | 38.4 |
| space3 | | | |
| Population - 2010 | 786 | 5,082 | 309,327,143 |
| Population - 2000 | 778 | 5,282 | 285,036,114 |
| Population - 1990 | 857 | 5,148 | 251,960,433 |
| Pop. 2000 to Now | -1.3% | -9.2% | 15.6% |
| Pop. 1990 to Now | -10.4% | -6.9% | 30.8% |
| space4 | | | |
| Population Density | 318.6 | 1,118.4 | 93.4 |
| Land Area | 2.2 | 4.4 | 3,531,905.4 |
| Water Area | 0.0 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Walnut, IA | Harlan, IA | United States |
| Age 0 to 4 | 5.5% | 4.9% | 5.9% |
| Age 5 to 9 | 8.9% | 6.1% | 6.1% |
| Age 10 to 14 | 3.4% | 3.0% | 6.6% |
| Age 15 to 17 | 1.3% | 4.2% | 3.9% |
| Age 18 to 20 | 1.2% | 4.1% | 4.0% |
| Age 21 to 24 | 4.4% | 3.7% | 5.2% |
| Age 25 to 34 | 14.3% | 10.7% | 13.8% |
| Age 35 to 44 | 4.5% | 9.4% | 12.9% |
| Age 45 to 54 | 9.5% | 11.9% | 12.6% |
| Age 55 to 59 | 7.4% | 7.0% | 6.7% |
| Age 60 to 64 | 7.9% | 9.7% | 6.3% |
| Age 65 to 74 | 20.3% | 12.4% | 9.6% |
| Age 75 to 84 | 6.7% | 7.5% | 4.5% |
| Age 85 and over | 4.8% | 5.3% | 1.9% |
| space1 | | | |
| Race | | | |
| | Walnut, IA | Harlan, IA | United States |
| White | 98.1% | 92.5% | 59.4% |
| Black | 1.6% | 2.5% | 12.2% |
| Asian | 0.0% | 0.0% | 5.6% |
| Native American | 0.0% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.0% | 0.4% |
| Two or More Races | 0.0% | 2.7% | 3.2% |
| Hispanic | 0.3% | 2.2% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Walnut, IA | Harlan, IA | United States |
| Married Population | 55.3% | 52.3% | 49.8% |
| Currently Married | 54.8% | 51.2% | 48.0% |
| Married but Separated | 0.5% | 1.1% | 1.8% |
| Single Population | 44.7% | 47.7% | 50.2% |
| Never Married | 19.5% | 26.9% | 33.8% |
| Divorced | 15.2% | 11.0% | 10.8% |
| Widowed | 9.9% | 9.8% | 5.6% |
| space5 | | | |
| Household Size | 2.11 | 2.12 | 2.60 |
| Households | 325 | 2,276 | 124,010,992 |
| Family Households | 175 | 1,256 | 80,755,759 |
| Married couple, w/children | 23.4% | 24.1% | 30.5% |
| Married couple, no children | 62.3% | 57.4% | 42.9% |
| Non Family Households | 150 | 1,020 | 43,255,233 |
| Single householder, w/children | 10.3% | 13.7% | 15.8% |
| Single householder, no children | 4.0% | 4.8% | 10.8% |