For mixed breed dogs, owners can use an individual’s weight to help determine how long he or she would be expected to live. In general, small dogs enjoy longer lives than do their larger counterparts.
A recent analysis of veterinary records revealed that dogs under 20 pounds had an average lifespan of 11 years while those over 90 pounds typically lived for only 8 years. Medium and large dogs fell in the middle at around 11 years. (State of Pet Health 2013 Report, Banfield Pet Hospital).

But average life expectancy isn’t the whole story. The very definition of “average” means that many individuals will have shorter lifespans while others can be expected to live much longer than the norm. Perhaps a better way to evaluate a dog’s longevity is to convert “dog years” into “human years.” In this way, we can understand just when a dog is an adult, a senior citizen, geriatric, or the equivalent of a human centenarian.