Is one “dog year” really equal to seven human years? Experts say no, it’s really just a rough rule of thumb. After all, there’s a huge variety of dog breeds, from Chihuahuas to Great Danes, which is the main reason why the lifespan of dogs differ more than the lifespan for any other animal. For example, a Chihuahua can live to 18 years old, while a Great Dane is lucky to make it to age eight.

Biologist Dr. Cornelia Kraus says that dogs are the most diverse mammals on the planet. Fully-grown animals can range in weight from two pounds to more than 300. And that weight difference matters in one key area, cancer. Since small dogs have fewer cells than big ones, they’re FIVE times less likely to develop a mutation that leads to cancer, so, they tend to live longer.

Of course, there are other factors that play a role in a dog’s lifespan that have nothing to do with size. In general, female dogs live longer than males, neutered animals live an average of 3 years longer than unneutered ones, and mutts usually live longer, because mixing the breeds often reduces the number of genetic diseases that can pop up in purebreds.

The best way to accurately assess how many years you can expect your dog to live is to see your vet. They can give you a pretty good estimate based on looking at their teeth, bones, muscles, and internal organs.  But the best way to ensure your dog lives as long as possible is to make sure they get plenty of exercise and keep their weight in check, just like with humans. 

And if you’re curious, you can get reasonably accurate age determination from an online calculator, like DogYears.com or on Pets.WebMD.com.