| | Decatur, AL | Clarksville, TN | United States |
| Population | 57,303 | 163,518 | 329,725,481 |
| Female Population | 50.7% | 49.8% | 50.5% |
| Male Population | 49.3% | 50.2% | 49.5% |
| Median Age | 40.6 | 30 | 38.4 |
| space3 | | | |
| Population - 2010 | 55,810 | 133,483 | 309,327,143 |
| Population - 2000 | 53,929 | 103,455 | 285,036,114 |
| Population - 1990 | 48,761 | 75,494 | 251,960,433 |
| Pop. 2000 to Now | 0.8% | 55.9% | 15.6% |
| Pop. 1990 to Now | 11.5% | 113.6% | 30.8% |
| space4 | | | |
| Population Density | 1,058.9 | 1,668.4 | 93.4 |
| Land Area | 54.1 | 98.0 | 3,531,905.4 |
| Water Area | 6.6 | 0.7 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Decatur, AL | Clarksville, TN | United States |
| Age 0 to 4 | 6.5% | 8.9% | 5.9% |
| Age 5 to 9 | 6.1% | 7.7% | 6.1% |
| Age 10 to 14 | 6.1% | 7.1% | 6.6% |
| Age 15 to 17 | 4.4% | 3.7% | 3.9% |
| Age 18 to 20 | 2.6% | 4.4% | 4.0% |
| Age 21 to 24 | 5.0% | 7.2% | 5.2% |
| Age 25 to 34 | 12.1% | 20.3% | 13.8% |
| Age 35 to 44 | 12.4% | 13.8% | 12.9% |
| Age 45 to 54 | 12.4% | 9.6% | 12.6% |
| Age 55 to 59 | 6.2% | 4.3% | 6.7% |
| Age 60 to 64 | 7.2% | 4.6% | 6.3% |
| Age 65 to 74 | 10.9% | 5.3% | 9.6% |
| Age 75 to 84 | 5.9% | 2.2% | 4.5% |
| Age 85 and over | 2.2% | 0.8% | 1.9% |
| space1 | | | |
| Race | | | |
| | Decatur, AL | Clarksville, TN | United States |
| White | 59.9% | 56.7% | 59.4% |
| Black | 22.6% | 21.8% | 12.2% |
| Asian | 0.4% | 2.6% | 5.6% |
| Native American | 0.1% | 0.3% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.3% | 0.2% |
| Other | 0.4% | 0.7% | 0.4% |
| Two or More Races | 2.5% | 5.9% | 3.2% |
| Hispanic | 14.1% | 11.7% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Decatur, AL | Clarksville, TN | United States |
| Married Population | 49.9% | 52.4% | 49.8% |
| Currently Married | 47.2% | 50.3% | 48.0% |
| Married but Separated | 2.6% | 2.1% | 1.8% |
| Single Population | 50.1% | 47.6% | 50.2% |
| Never Married | 28.3% | 30.2% | 33.8% |
| Divorced | 13.3% | 13.6% | 10.8% |
| Widowed | 8.5% | 3.9% | 5.6% |
| space5 | | | |
| Household Size | 2.37 | 2.69 | 2.60 |
| Households | 23,797 | 59,481 | 124,010,992 |
| Family Households | 14,199 | 41,036 | 80,755,759 |
| Married couple, w/children | 25.6% | 35.3% | 30.5% |
| Married couple, no children | 46.0% | 35.2% | 42.9% |
| Non Family Households | 9,598 | 18,445 | 43,255,233 |
| Single householder, w/children | 17.9% | 20.3% | 15.8% |
| Single householder, no children | 10.6% | 9.3% | 10.8% |