“How old is your dog?” It’s a common question and you’ve likely answered along the lines of, “He’s 6, or 42 in dog years.” The common perception has been that one human year equals seven dog years, meaning for each calendar year, a dog’s body ages the equivalent of seven. Although this calculation has perpetuated for over half a century, it’s actually a myth.
Since at least the thirteenth century, people have tried to equate a dog’s lifespan to that of a human, noting at various times that humans live to 81 or 90 years, while dogs live to 9 or 10 years, respectively. That 9-to-1 ratio became 7-to-1 in the 1950’s, possibly to encourage annual veterinarian visits. This estimation was almost immediately challenged by scientists, including a French researcher who compared aging in humans and dogs based on common life markers like puberty and age-related illnesses. His studies suggested a year-old dog was the equivalent of a 15-year-old human, a 2-year-old dog equal to a 24-year-old person and that each following year was equal to six human years.
Today’s researchers believe breed and size influence the human-to-dog age comparison. Four breed size categories help to better approximate canine aging – small, medium, large and giant. Research on specific breeds shows that some defy the expected age ratio based on their size, including poodles, beagles, golden retrievers and Great Danes. The growth hormone IGF-1 is believed to speed up aging when found in higher concentrations (as with larger dogs).
Human years to dog years equate as follows, by breed size:
Small (up to 20 pounds) = 15:1, 8:1 for the second year, 5:1 for the third year, and 4:1 for each year after the third;
Medium (21 to 50 pounds) = 15:1, 9:1 for the second year, and 4:1-5:1 for each year after the second;
Large (51 to 90 pounds) = 14:1 for the first year, 8:1 for the second year, 7:1 for the third year, and 5:1-6:1 for each year after the third;
Giant (over 90 pounds) = 14:1 for the first year, 6:1 for the second year, and 7:1-8:1 for every year after the second.