For the most part, gray spelled with an “a” is more commonly used in American English (think “a” for “American”). Grey, spelled with an “e” is said to be more commonly used in the English language outside of the United States. Both of the words have the same meaning although they are spelled differently.

According to the website Grammarist, “In the U.K… grey appears about twenty times for every instance of gray. In the U.S. the ratio is reversed.”

Grammarist also points out a couple of exceptions to the words being interchangeable, “Greyhound, for the breed of dog, always has an e, while grayling, which refers to several types of fish, always has an a.”