St. Augustine, IL has a population of 115 and is the 26,545th largest city in the United States. The population density is 192 per sq mi which is 14% lower than the Illinois average and 112% higher than the national average. The median age in St. Augustine is 56 which is approximately 49% higher than the Illinois average of 37. In St. Augustine, 65% of the population over 15 years of age are married, 100% speak English and 0% speak Spanish. 70% of St. Augustine residents were born in Illinois, 30% were born out of state, 0% were born outside of the United States and 0% were foreign born.