How are dog years calculated?

Written and fact-checked by The Editors of Encyclopaedia Britannica

Many people have learned that one human year equals roughly seven dog years. However, this is a huge oversimplification. How a dog ages is based largely on its size and breed. Smaller dogs tend to age “slower” than larger dogs. Also, the aging rate of some breeds, such as beagles, differs from that of other similarly sized breeds.