Diagonal, IA has a population of 319 and is the 22,146th largest city in the United States. The population density is 353 per sq mi which is 546% higher than the Iowa average and 290% higher than the national average. The median age in Diagonal is 55 which is approximately 45% higher than the Iowa average of 38. In Diagonal, 68% of the population over 15 years of age are married, 98% speak English and 2% speak Spanish. 74% of Diagonal residents were born in Iowa, 24% were born out of state, 0% were born outside of the United States and 1% were foreign born.