Determining the expected lifespan of a dog requires finding the average lifespan of that dog's breed, then adjusting for factors such as the animal's health, weight and environment. For mixed breed dogs, use the dog's weight to get an idea of its average lifespan, then adjust accordingly.
Most data about canine lifespans suggests that smaller dogs, such as Yorkshire terriers and Chihuahuas, have longer lifespans than Mastiffs, Great Danes and other large breed dogs. The relationship between weight and maximum lifespan also holds true for mixed breed dogs. However, some dogs buck this trend, as the Australian Shepherd is a medium-sized dog with one of the longest average lifespans at up to 18 years, while the tiny Pomeranian has an average lifespan of only 12 years.
Keep in mind that the average lifespan of a breed or weight class of dogs represents data from the lifespans of many dogs of that group. Health factors, such as obesity, injury and disease, can have a negative impact on a dog's potential lifespan. A dog's environment also has a large effect on its lifespan, as dogs kept in clean, spacious and stable homes have longer lifespans than dogs kept in adverse conditions.
Even before taking health and environmental factors into account, lifespan varies from dog to dog, even within the same breed or weight class. Just as in humans, some dogs live longer or shorter lives independent of other factors.