Crystal Lake, IL has a population of 40,475 and is the 1,021st largest city in the United States. The population density is 2,125 per sq mi which is 856% higher than the Illinois average and 2246% higher than the national average. The median age in Crystal Lake is 39 which is approximately 4% higher than the Illinois average of 37. In Crystal Lake, 59% of the population over 15 years of age are married, 85% speak English and 11% speak Spanish. 66% of Crystal Lake residents were born in Illinois, 22% were born out of state, 1% were born outside of the United States and 11% were foreign born.