Fuzzy Math: How Do “Dog Years” Work?

My next-door neighbors have a little terrier named Melvin (not pictured). My old neighbor had a very big Bernese mountain dog (also not pictured). I didn’t know its name, and usually just referred to it as The Big Dog or The Beast. Ten Melvins, soaking wet and carrying a few dollars in change in their mouths, would probably weigh about the same as The Big Dog. The Big Dog could probably eat all ten of those Melvins, and the change, and still be hungry.

In other words, no two dog breeds are created equal, even if all dogs go to heaven. They come in different shapes and sizes. They reach maturity at different ages and time takes its toll on their bodies differently. On average, smaller dogs like Melvin have longer lifespans (14-16 years) than big ones (12-13 years), which have longer lifespans than certain “giant breeds” like the Bernese (7-10 years).

Age is an important piece of social information and can tell you a number of things about a person. Since dogs are social creatures and are part of many of our social circles, it’s nice to have that information about them, too. With the way different breeds mature and age at different rates, though, things can get confusing. A five-year-old terrier isn’t at the same point in its life as a five-year-old Great Dane. In fact, the two dogs are pretty much at opposite ends of their lifespans.

To work around this snag, people have long followed the rule that “dog year” is equal to seven human years and multiplying a dog’s age by seven gives you its age in the context of the human lifespan. The x7 trick is easy enough to do, but it’s no more helpful than just giving the dog’s age in dog years.

A five-year-old terrier and a five-year-old Great Dane are not only unlike each other in age, neither is similar to a 35-year-old human.

The dog year to human year conversion is especially inaccurate with very young dogs. A dog that’s one year old would be seven years old in human years. Most dog breeds are sexually mature and fully capable of reproducing when they’re one year old, while most seven-year-old humans are not. One just wants a puppy, but the other can make puppies.

Crafting a simple, proportional relationship between Rover’s age and yours might seem convenient, but it’s not doing anyone good. While no one formula for dog-to-human age conversion is scientifically agreed on, we can at least say that there are some better alternatives to multiplying by seven.

New Math

The folks at The Dog Guide suggest that when we think about “dog years,” we have to consider the breed and calculate accordingly. Across the board, they say, you can consider the first year of a dog’s life as equivalent to 15 or so human years. By that time, dogs and humans are approaching their adult size and have reached sexual maturity. On their 2nd birthday, you should add about 3-8 more years to your dogs “human age,” depending on size, and value each dog year as being worth 4-5 human years from that point on.

Stanley Coren, psychologist and author of How Dogs Think, The Pawprints of History and other dog books, uses a similar system. The way he sees it, one-year-old dogs and two-year-old dogs are like 16-year-old people and 24-year-old people, respectively, regardless of breed. For the next three years, each birthday tacks on five human years, and from age five and beyond, everything depends heavily on the dog’s size. Coren suggests adding four years of human life to the age of small dogs after age 5, and six years for large breeds.

When you’re crunching these numbers for your own dog, you should also keep in mind individual characteristics like weight, diet, exercise and veterinary care. If you have a terrier that weighs the same as a Bernese mountain dog, it’s a safe bet that the little porker’s lifespan is going to be decreased. If you keep your dog healthy, lean and on a quality diet, though, then they’ll skew a little younger in human years when the conversion system provides a little wiggle room.