Buffalo, IL has a population of 503 and is the 19,503rd largest city in the United States. The population density is 1,377 per sq mi which is 519% higher than the Illinois average and 1419% higher than the national average. The median age in Buffalo is 46 which is approximately 23% higher than the Illinois average of 37. In Buffalo, 60% of the population over 15 years of age are married, 98% speak English and 2% speak Spanish. 91% of Buffalo residents were born in Illinois, 9% were born out of state, 0% were born outside of the United States and 0% were foreign born.