Many people assume that calculating a dog's age in human years is as easy as multiplying the dog's age by seven, but it's not quite that simple.

Dogs mature faster than we do, and the first year of a canine's life is typically equivalent to about 15 human years.

However, size and breed play a key role in how quickly a canine ages, and as a general rule, small dogs live longer than large ones.

Big dogs, those weighing more than 50 pounds like Great Danes, take longer to reach adulthood than Chihuahuas, for example. However, while Great Danes may reach middle age at 6, dogs that weigh less than 20 pounds won't reach that point for another few years.

In other words, small dogs mature faster and experience a longer adulthood.

Medium-sized dogs — those that weigh 20 to 50 pounds — tend to fall in the middle.

In nature, larger animals tend to outlive their smaller counterparts. For example, elephants can live up to 70 years while mice average only 4. But dogs' lifespans differ because they've been bred to come in a wide variety of sizes.

"It's possible that by creating all of these diversely sized dogs that we unmasked this aging phenomenon," Kate Creevy, a professor of internal medicine at the University of Georgia, told BBC.

When researchers at the University of Göttingen in Germany analyzed ages of death in 74 different breeds, they discovered that an increase of 4.4 pounds in body mass led to a loss of about one month of life expectancy.

Clearly, there's a lot more to calculating a dog's age in human years than simply multiplying by seven, but if you're looking for a relatively easy way to determine your pup's age, dog behaviorist Cesar Milan suggests this method: Subtract two from the age, multiply that by four and add 21.

Or you could simply use Pedigree's dog-age calculator.

German shepherd and German shepherd puppy. (Photo: Svetlana Valoueva/Shutterstock)

Not sure how old your dog is?

If you've adopted a puppy or older dog and aren't sure of the animal's history, talk to your veterinarian. Upon completing a physical examination of the dog, your vet will have a good idea of the animal's health and age.

However, there are other ways to estimate your pet's age:

5-8 weeks: At this age, a puppy's eyes are open and it starts to get teeth. By 8 weeks, all the teeth are in.

16 weeks-8 months: Baby teeth are falling out and being replaced by permanent teeth. By 8 to 12 months old, all the permanent teeth are in.

1-2 years: Most breeds have reached their full height by this time, and teeth may be duller and show some yellowing.

2-3 years: Teeth will have tartar buildup and show some wear. The teeth may also show signs of gingivitis if they aren't regularly brushed.

3-7 years: Teeth and gums will look more worn, and the dog may develop gray hairs around the muzzle.

Over 7 years: Heavy tartar buildup is likely and some teeth may be missing. The dog's eyes may become cloudy due to cataracts, and the animal may also have a raspier bark and decreased mobility.

Laura Moss writes about a variety of topics with a focus on animals, science, language and culture. But she mostly writes about cats.

The truth about how dogs age
Multiplying your dog's age by 7 won't give you an accurate idea of its age in human years.