Pet Myths: Dog Years to Human Years

3 min read


By Megan Sullivan

There is a long-standing belief that one dog year is the equivalent of seven human years. But does this add up? What does a dog year truly mean?

According to researchers, dog trainers, and veterinarians, the math isn’t that straightforward.

A dog’s average life span is roughly one-seventh that of humans, which could explain how this myth came about, says Dr. Katy Nelson, a veterinarian at the Belle Haven Animal Medical Centre in Washington, D.C., and a medical advisor for petMD. “It makes sense how people have come to that conclusion,” she says, “but it’s not necessarily accurate.”

No matter how you crunch the numbers, dogs age faster than humans. During the first two years of their lives, dogs grow and mature rapidly, says Dr. Kathryn McGonigle, clinical associate professor of internal medicine at Penn Vet. “You may have a pet who goes through infancy, pre-teens, and teens and gets into his young adulthood by about 2,” she says. This means a dog’s first two years are equal to about 15 to 24 human years. From there, the aging process slows down considerably. “After that, we think each year of their life is roughly equivalent to maybe four or five of our years.”

Once a dog reaches middle age, however, other factors come into play. Think of dog aging as more of a sliding scale, describes petMD trainer Victoria Schade. “There is no exact science here, but once they get to be about 6 years old, then the size, breed, and type of dog all start to come into play as to how quickly they’re going to age,” she says. 

Small dogs have slower aging rates than their larger counterparts. “Our smaller dogs have a longer life span,” Nelson says. Pint-sized pups like Chihuahuas and Yorkies may live to be 15 to 18 years old, she says, whereas the life span of large and giant-breed dogs like Great Danes and Mastiffs can be as little as 7 to 9 years. “Middle-sized dogs go anywhere in between.”

So, why don’t dogs live as long as humans? The disparity in life span comes down to genetics, McGonigle says. Like all living things, humans and dogs are made up of millions of cells. When scientists study the cause of aging in humans, they talk about cell damage and genetic programming. “There’s something in the genetics that dictates how long those cells are going to survive and function normally,” she explains.

But genes aren’t the only indicator of a dog’s probable life span. “Absolutely, other factors like environment and diet can play into it, just like it does with us,” McGonigle says.

While the notion that one dog year equals seven human years is a myth, the fact remains that dogs have shorter life spans than we do. How fast your canine companion ages will largely depend on his overall health, breed status, and size, Nelson reiterates. 

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