| | Onida, SD | Agar, SD | United States |
| Population | 580 | 95 | 329,725,481 |
| Female Population | 46.2% | 43.2% | 50.5% |
| Male Population | 53.8% | 56.8% | 49.5% |
| Median Age | 47.9 | 62.1 | 38.4 |
| space3 | | | |
| Population - 2010 | 663 | 75 | 309,327,143 |
| Population - 2000 | 740 | 82 | 285,036,114 |
| Population - 1990 | 761 | 82 | 251,960,433 |
| Pop. 2000 to Now | -15.1% | -2.4% | 15.6% |
| Pop. 1990 to Now | -17.5% | -2.4% | 30.8% |
| space4 | | | |
| Population Density | 863.1 | 463.4 | 93.4 |
| Land Area | 0.7 | 0.2 | 3,531,905.4 |
| Water Area | 0.0 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Onida, SD | Agar, SD | United States |
| Age 0 to 4 | 8.1% | 0.0% | 5.9% |
| Age 5 to 9 | 6.2% | 1.1% | 6.1% |
| Age 10 to 14 | 7.4% | 0.0% | 6.6% |
| Age 15 to 17 | 3.8% | 0.0% | 3.9% |
| Age 18 to 20 | 0.9% | 0.0% | 4.0% |
| Age 21 to 24 | 3.3% | 0.0% | 5.2% |
| Age 25 to 34 | 11.0% | 22.1% | 13.8% |
| Age 35 to 44 | 7.1% | 12.6% | 12.9% |
| Age 45 to 54 | 11.6% | 3.2% | 12.6% |
| Age 55 to 59 | 8.4% | 10.5% | 6.7% |
| Age 60 to 64 | 10.9% | 8.4% | 6.3% |
| Age 65 to 74 | 7.1% | 38.9% | 9.6% |
| Age 75 to 84 | 12.4% | 1.1% | 4.5% |
| Age 85 and over | 1.9% | 2.1% | 1.9% |
| space1 | | | |
| Race | | | |
| | Onida, SD | Agar, SD | United States |
| White | 92.9% | 100.0% | 59.4% |
| Black | 0.0% | 0.0% | 12.2% |
| Asian | 0.0% | 0.0% | 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 | 3.6% | 0.0% | 3.2% |
| Hispanic | 3.4% | 0.0% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Onida, SD | Agar, SD | United States |
| Married Population | 56.6% | 40.4% | 49.8% |
| Currently Married | 56.6% | 40.4% | 48.0% |
| Married but Separated | 0.0% | 0.0% | 1.8% |
| Single Population | 43.4% | 59.6% | 50.2% |
| Never Married | 24.2% | 27.7% | 33.8% |
| Divorced | 13.4% | 21.3% | 10.8% |
| Widowed | 5.7% | 10.6% | 5.6% |
| space5 | | | |
| Household Size | 2.25 | 1.42 | 2.60 |
| Households | 258 | 67 | 124,010,992 |
| Family Households | 137 | 20 | 80,755,759 |
| Married couple, w/children | 19.7% | 10.0% | 30.5% |
| Married couple, no children | 71.5% | 90.0% | 42.9% |
| Non Family Households | 121 | 47 | 43,255,233 |
| Single householder, w/children | 7.3% | 0.0% | 15.8% |
| Single householder, no children | 1.5% | 0.0% | 10.8% |