Underwood, IA has a population of 808 and is the 16,546th largest city in the United States. The population density is 736 per sq mi which is 1248% higher than the Iowa average and 713% higher than the national average. The median age in Underwood is 34 which is approximately 12% lower than the Iowa average of 38. In Underwood, 69% of the population over 15 years of age are married, 99% speak English and 1% speak Spanish. 62% of Underwood residents were born in Iowa, 38% were born out of state, 0% were born outside of the United States and 1% were foreign born.