Instructions
Add 14 years to any size dog's age after the first year of life. For instance, a 1-year-old Jack Russell Terrier, a relatively small dog, and a 1-year-old Great Dane, a big dog, would both be 15 human years on their first birthday.
Add four more years to the dog's age for each year of life for the next four years, or until the dog is five years old. This rule applies for dogs of any size. For instance, a dog that is 3 years old is now considered 28 in human years.
Continue to add four "human" years to the dog age for every year of its life for smaller breed dogs, or dogs that are less than 20 pounds, according to WebMD. As medium dog's become older, approximately five years are added for every year of its life. Larger breeds, which are dogs over 50 pounds, will also have five years added for each year of its life until the dog reaches 16 years of age, at which time large chunk of years are added to its age and the dog is considered 120 in human years.
How to Calculate a Dog's Birth Date Age
It is a common misconception that dogs age at a rate of seven years human years for every one year of its life, according to WebMD (see reference 2). In actuality, there are several factors that determine how quickly a dog ages, the most important being its size. Determining a dog's age by its size and a few set of calculations will help an owner find out how old its dog is in "human" years.