Decatur, AL has a population of 55,419 and is the 722nd largest city in the United States. The population density is 1,025 per sq mi which is 1014% higher than the Alabama average and 1031% higher than the national average. The median age in Decatur is 39 which is approximately equal to the Alabama average of 39. In Decatur, 57% of the population over 15 years of age are married, 88% speak English and 11% speak Spanish. 65% of Decatur residents were born in Alabama, 27% were born out of state, 1% were born outside of the United States and 7% were foreign born.