Waterloo, WI has a population of 3,341 and is the 8,281st largest city in the United States. The population density is 881 per sq mi which is 907% higher than the Wisconsin average and 872% higher than the national average. The median age in Waterloo is 37 which is approximately 5% lower than the Wisconsin average of 39. In Waterloo, 59% of the population over 15 years of age are married, 88% speak English and 10% speak Spanish. 78% of Waterloo residents were born in Wisconsin, 13% were born out of state, 0% were born outside of the United States and 9% were foreign born.