New Castle, DE has a population of 5,372 and is the 6,158th largest city in the United States. The population density is 1,580 per sq mi which is 214% higher than the Delaware average and 1644% higher than the national average. The median age in New Castle is 46 which is approximately 16% higher than the Delaware average of 40. In New Castle, 53% of the population over 15 years of age are married, 86% speak English and 13% speak Spanish. 52% of New Castle residents were born in Delaware, 41% were born out of state, 4% were born outside of the United States and 4% were foreign born.