Waterloo, IA has a population of 68,357 and is the 537th largest city in the United States. The population density is 1,113 per sq mi which is 1938% higher than the Iowa average and 1129% higher than the national average. The median age in Waterloo is 37 which is approximately 4% lower than the Iowa average of 38. In Waterloo, 52% of the population over 15 years of age are married, 89% speak English and 5% speak Spanish. 74% of Waterloo residents were born in Iowa, 18% were born out of state, 1% were born outside of the United States and 7% were foreign born.