| | Barnwell, SC | Allendale, SC | United States |
| Population | 4,526 | 2,827 | 329,725,481 |
| Female Population | 56.2% | 53.6% | 50.5% |
| Male Population | 43.8% | 46.4% | 49.5% |
| Median Age | 32.4 | 37 | 38.4 |
| space3 | | | |
| Population - 2010 | 4,764 | 3,455 | 309,327,143 |
| Population - 2000 | 5,035 | 4,052 | 285,036,114 |
| Population - 1990 | 5,255 | 4,410 | 251,960,433 |
| Pop. 2000 to Now | -15.4% | -30.2% | 15.6% |
| Pop. 1990 to Now | -18.9% | -35.9% | 30.8% |
| space4 | | | |
| Population Density | 577.2 | 854.1 | 93.4 |
| Land Area | 7.8 | 3.3 | 3,531,905.4 |
| Water Area | 0.1 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Barnwell, SC | Allendale, SC | United States |
| Age 0 to 4 | 7.9% | 6.5% | 5.9% |
| Age 5 to 9 | 7.4% | 8.8% | 6.1% |
| Age 10 to 14 | 7.4% | 8.9% | 6.6% |
| Age 15 to 17 | 2.5% | 3.0% | 3.9% |
| Age 18 to 20 | 7.4% | 3.1% | 4.0% |
| Age 21 to 24 | 8.4% | 3.9% | 5.2% |
| Age 25 to 34 | 11.8% | 10.0% | 13.8% |
| Age 35 to 44 | 9.4% | 9.4% | 12.9% |
| Age 45 to 54 | 12.3% | 10.4% | 12.6% |
| Age 55 to 59 | 5.0% | 5.4% | 6.7% |
| Age 60 to 64 | 4.9% | 8.1% | 6.3% |
| Age 65 to 74 | 8.0% | 14.3% | 9.6% |
| Age 75 to 84 | 5.2% | 7.1% | 4.5% |
| Age 85 and over | 2.3% | 1.0% | 1.9% |
| space1 | | | |
| Race | | | |
| | Barnwell, SC | Allendale, SC | United States |
| White | 40.9% | 15.5% | 59.4% |
| Black | 57.6% | 80.6% | 12.2% |
| Asian | 0.0% | 0.2% | 5.6% |
| Native American | 0.3% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.4% | 0.0% | 0.4% |
| Two or More Races | 0.4% | 0.3% | 3.2% |
| Hispanic | 0.4% | 3.3% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Barnwell, SC | Allendale, SC | United States |
| Married Population | 29.6% | 38.0% | 49.8% |
| Currently Married | 26.3% | 29.5% | 48.0% |
| Married but Separated | 3.3% | 8.4% | 1.8% |
| Single Population | 70.4% | 62.0% | 50.2% |
| Never Married | 49.6% | 38.7% | 33.8% |
| Divorced | 12.0% | 2.6% | 10.8% |
| Widowed | 8.8% | 20.7% | 5.6% |
| space5 | | | |
| Household Size | 2.40 | 2.41 | 2.60 |
| Households | 1,819 | 1,160 | 124,010,992 |
| Family Households | 968 | 611 | 80,755,759 |
| Married couple, w/children | 17.7% | 3.6% | 30.5% |
| Married couple, no children | 25.6% | 48.3% | 42.9% |
| Non Family Households | 851 | 549 | 43,255,233 |
| Single householder, w/children | 32.0% | 25.0% | 15.8% |
| Single householder, no children | 24.7% | 23.1% | 10.8% |