A dog year is equal to seven chronological years. The "dog year" was invented to describe how dogs age faster than humans. The practical use of the dog year, however, is limited because it is an oversimplification of reality.
For example, a year-old dog is biologically equivalent to a 15-year-old human, not a 7-year-old human. The rate of development and aging of a dog is also strongly influenced by its breed and size. Small dogs mature faster than large dogs, but also tend to live longer.
A 5-year-old dog, regardless of size, is approximately biologically equal to a 36 year old human. A 15-year-old small dog is equal to a 76-year-old human, but a 15-year-old large dog is equal to a 93-year-old human.