Spencer, IA has a population of 11,172 and is the 3,620th largest city in the United States. The population density is 1,022 per sq mi which is 1771% higher than the Iowa average and 1028% higher than the national average. The median age in Spencer is 41 which is approximately 8% higher than the Iowa average of 38. In Spencer, 65% of the population over 15 years of age are married, 97% speak English and 3% speak Spanish. 70% of Spencer residents were born in Iowa, 28% were born out of state, 0% were born outside of the United States and 2% were foreign born.