The life expectancy of any dog is commonly expressed in terms of dog years. It depends entirely on the dog's breed, and size as well as the dog's surroundings. Various other factors affect the life expectancy of a dog. A dog's life span changes according to the diet provided to him and also by the number of medical problems the dog tends to face over the years.

The average life span of the small dog like Chihuahua is about 15 to 16 years, while medium sized dogs like Border Collies have a life span of 10 to 13 years. It can be noticed big dogs like Great Dan live up to only 7 to 8 years. Calculating the life expectancy of a particular dog can be accurately done with the help of "size or the specific breed calculator," which is said to be one of the most accurate age approximate results about the dog's predictable lifespan based on the size as well as the breed of the dog.

It is generally assumed that, on an average, "one human year equals to seven dog years." This statement may not be appropriate, since the first two years are jointly represented by 18-25 years in such schemes and because the entire ratio cannot be generalized for all the dogs though, they belong to different sizes and breeds. An exact ratio of dog years vs. human year cannot be formulated. However, some obvious features remain similar between both aging humans and dogs.

It is generally assumed that the human equivalent of a one-year dog is supposedly fully grown by both mental and physical aspects. Dogs like humans tend to develop advanced muscular features as they grow along with a similar mental development. Through a major dog study, it has been estimated for dogs of all breeds and sizes that, nearly "64 percent of dogs" were etherized because they were affected with some kind of diseases. Only 8 percent of the total dogs live beyond 15 years and nearly 16 percent of them die yearly due to heart diseases and cancer. Human lives are not terminated even if they suffer from rare and incurable diseases' and this is the main reason why the lifespan of an average human being is reasonably long.

The effects of aging in dogs is said to be dormant for several years. But like humans, dogs do undergo physical changes as well. The first sign of aging in dogs is said to be the decrease in its activity levels. The sleeping period in these dogs also becomes longer. The canines experience a drastic change in their skin condition, limb usage, appetite and they experience tooth loss as well. Their sense of hearing and vision also diminishes gradually. Older dogs tend to experience stiffness in their body. All these obvious signs of aging can even be found in humans. The human years are extended only because of advanced modern medicine and various other drugs that are designed to deal with symptoms of old age.

Dogs and humans may not have the same physiology, yet some obvious similarities can be noticed when both species age. Dog Years and Human Years may not be directly related by some advanced equation, but to the naked eye the equivalence of corresponding features of Dog Years and Human Years are quite visible.