They say age is just a number. Well when it comes to calculating dog years, that number is more complicated than I realized. Dogs, I have good news and bad news. The good news is the years may not be flying by as quickly as we thought. Contrary to what we’ve been led to believe, one dog year is NOT the equivalent of seven human years. According to the American Veterinary Medical Association, calculating dog years is a lot more complicated. In general, the first year of a medium size dog’s life is equal to 15 human years! Then by the second year we slow down, that during our terrific twos we cover approximately nine human years. Then after that, it is roughly one dog year equals five human years. But even this equation doesn’t accurately explain the ratio of dog years to human. The bad news is apparently size does matter…a lot! Small dogs age a lot slower than large dogs.

In “The size-life span trade-off decomposed: Why large dogs die young,” published in The American Naturalist (April 2013), Cornelia Kraus, an evolutionary biologist at the University of Göttingen in Germany, and her colleagues found that large dogs age at an accelerated pace, which may in turn lead to a shorter lifespan. After studying 50,000 deaths in 74 breeds of dogs, based on their calculations, which are way too complicated to explain here, Kraus concluded, “that every 4.4 pounds of body mass reduced a dog’s life expectancy by about a month.” Or as Professor Mark Elgar, of the University of Melbourne’s School of Biosciences, later explained, “a larger dog, because of its size, may put more strain on its physiological processes, meaning they tend to wear out more quickly.”

However, more research remains to be done to fully understand why dogs age differently based on size. Some theorize that because large breed dogs age faster, they also develop age-related diseases earlier in their lives compared to smaller dogs. Their rapid growth from a small puppy to a large dog comes with increased free-radical activity, and a faster rate of growth in abnormal cells which in turn cause cancer. Another theory is that small dogs tend to be more “pampered,” such as being carried more or kept inside. As opposed to large dogs, who tend to lead more active lifestyles, hiking, running, swimming, and participating in other sports. That active lifestyle comes with a higher risk for injuries, which in turn could lead to an earlier mortality. But humans, before you put your pup in a bubble, remember this is all speculation at this point. Scientists are continuing to examine aging in dogs, through comprehensive studies such as the Dog Aging Project, Morris Animal Foundation’s Golden Retriever Lifetime Study, and the Foundation’s offshoot of this study the Golden Oldies Project.

Now to get back to this new way of calculating age. After studying 104 Labrador Retrievers, in 2019, researchers at the University of California San Diego came up with a more accurate way to replace the previous 7:1 theory. In their article “Quantitative translation of dog-to-human aging by conserved remodeling of epigenetic networks,” published by bioRxiv, they developed this new way of calculating a dog’s age based on changes to DNA over time. As explained by the AKC, “the results allowed them to derive a formula for adjusting dogs’ ages to ‘human years,’ by multiplying the natural logarithm of the dog’s age by 16 and adding 31 (human_age = 16ln(dog_age) + 31).” 

Ok, to be honest, I have no idea how this formula works because when I tried it, Asa came out to be 143 years old, and he’s way too spry to be over 100 in human years! Although Asa insists if you use the natural logarithm to solve for “In,” the formula does work. Whatever! Based on their chart, as a 72 lbs Golden Retriever, at Asa’s current age of 7, he’s closer to being 50 in human years. Whereas as our Terrier neighbors who are under 10 lbs at the same age are closer to 44 years old. But best yet, when I turn 10 at the end of the month, based on this new formula, rather than being the equivalent of 70 based on the old formula, I’m going to be a mere 66 in human years!

Now to get back to that 7 to 1 ratio of dog to human years, where did that theory originate? According to Kelly M. Cassidy, a curator of the Charles R. Connor Museum at Washington State University, the 7:1 ratio dates back to the 1950s. It is based on the premise that humans live to approximately age 70, and dogs to 10, which conveniently works out to 7:1. Expanding on that theory, William Fortney, a veterinarian at Kansas State University, believes it originated as a “marketing ploy,” to encourage owners to bring their dogs in for an annual exam, which is important to do regardless of if you’re using the old 7:1 formula or the more complicated modern calculation. I’m just content in knowing that as my 10th birthday approaches at the end of the month, I will be 66 in dog years rather than 70!