According to the United States Census Bureau, the CDP has a total area of 16.5 square miles (43 km2), of which 16 sq mi (41 km2) is land and 0.5 sq mi (1.3 km2) (3.15%) is water.

The 2020 United States census counted 18,423 people, 8,227 households, and 5,037 families in Westerly.

There were 9,195 housing units at an average density of 580.7 per square mile (224.2/km2).

34.4% of households consisted of individuals and 16.4% had someone living alone who was 65 years of age or older.

[8] The percent of those with a bachelor’s degree or higher was estimated to be 27.6% of the population.

[12][13] As of the census[14] of 2000, there were 17,682 people, 7,346 households, and 4,657 families residing in the CDP.

30.6% of all households were made up of individuals, and 13.5% had someone living alone who was 65 years of age or older.