Plainfield, IL has a population of 41,881 and is the 978th largest city in the United States. The population density is 1,717 per sq mi which is 672% higher than the Illinois average and 1795% higher than the national average. The median age in Plainfield is 34 which is approximately 8% lower than the Illinois average of 37. In Plainfield, 65% of the population over 15 years of age are married, 83% speak English and 8% speak Spanish. 70% of Plainfield residents were born in Illinois, 20% were born out of state, 0% were born outside of the United States and 10% were foreign born.