Crystal Lake, IL has a population of 40,598 and is the 989th largest city in the United States. The population density is 2,224 per sq mi which is 862% higher than the Illinois average and 2354% higher than the national average. The median age in Crystal Lake is 38 which is approximately 2% higher than the Illinois average of 37. In Crystal Lake, 61% of the population over 15 years of age are married, 84% speak English and 10% speak Spanish. 67% 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.