Smithfield, IL has a population of 172 and is the 25,104th largest city in the United States. The population density is 370 per sq mi which is 67% higher than the Illinois average and 309% higher than the national average. The median age in Smithfield is 46 which is approximately 22% higher than the Illinois average of 37. In Smithfield, 76% of the population over 15 years of age are married, 99% speak English and 1% speak Spanish. 85% of Smithfield residents were born in Illinois, 15% were born out of state, 0% were born outside of the United States and 1% were foreign born.