Destin Benoit
Your guide to this review today is by pet expert Destin Benoit
Published December 30, 2020

We wish our pooches could live forever, but like humans, your dog will age, but at a faster rate. So, working out your dog’s age in human years is a good way to understand exactly which life stage your pet is at. For years, the 1:7 rule – that’s 1 dog year for every 7 human years – has been used to age our canine pets. But taking into account your dog’s size, breed, and life- stage, it turns out the math isn’t as straightforward as that.

So, how do you calculate dog years to human years? We take a look at the theories that really add up.

Why the 1:7 Theory is Wrong

While it’s still widely used, the one dog year for every seven human years theory is actually not supported by science. Thought to have started back in the 1950s, it has, however, become the traditional way to work out a dog’s human age based on the belief that on average humans live to 70, while dogs live to around 10 – hence the 1:7 theory.

As a rough guide, the 1:7 rule has been useful in focusing pet owners on the faster aging pattern of their pet and helped veterinarians to promote the need to get the right care for dogs. And, as a simple math formula, it’s easy and convenient to work out. But it is not an accurate calculation of dog years to human and doesn’t reflect the differing aging rates and life expectancy in dogs, due to size, breed, genetics and life stage.

funny chihuahua dog posing on a beach in sunglasses

Why is Understanding My Dog’s Age So Important?

Knowing your dog’s age will enable you to give them the very best life you can, whether they are a young pup or a silver-jowled senior. And, as your pooch will age faster than we humans, then knowing where they are at any point on their life cycle will ensure you can give them the right age-appropriate diet, exercise, medical and general care to keep them healthy and happy.

The Pattern of Your Dog’s Aging

When it comes to aging, not all dogs are equal, so having a one size fits all formula is not the most accurate way to know your dog’s ‘human age’ and life span. When working out your dog’s pattern of aging throughout their life, there are key factors you need to take into account before trying to relate their age to equivalent human years.

Puppies mature more quickly during their earlier years  

One of the key factors that skews the uniform 1:7 formula is that dogs actually develop – and so age – at a quicker rate during their first two years of life, than they do at any other life stage. The rate of canine development depends on DNA methylation, which is nonlinear in dogs, compared to humans. And so it’s thought that the ratio to dog and human years during the first year of a pup’s life is one dog year to 15 human years (1:15), dropping to around nine years (1:9) as a one-two year old. After 2 years, their development levels out, with the dog to human year ratio evening out to around four or five for each of their subsequent years. So, as you can see, these ratios don’t fit with the uniform pattern of 1:7.

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Size counts 

As well as early years development, the size of your dog also influences the rate at which they age, when it comes to dog to human years. Scientists and experts are exploring this, as research has yet to fully explain why there is a correlation between a dog’s body mass and their life expectancy. Large dogs are known to age at a faster pace than small dogs, with research scientists indicating that for every 4.4 pounds of body mass, a dog’s life span will be reduced by around one month. Reasons for this could be that the larger the dog, the more they can be affected by age-related health issues and diseases as well as problems associated with abnormal cell growth. And, just to complicate it further, smaller dogs such as the dachshund age much faster in those first few years of life, compared to large breeds, for example, the Labrador retriever, but then slow down to live longer.

Your dog’s breed plays a role

As well as size, the breed of your dog also influences their lifespan, with small and toy dogs typically living longer than large or giant breeds. So, you can expect a small dog like a Jack Russell not to reach their senior years until around the age of 10, whereas a giant breed such as a Great Dane will be considered seniors from around seven.

Genetics and DNA

Researchers have shown that a dog’s genes and DNA will also influence their lifespan, with pure breeds in particular, showing a decrease in life expectancy, the larger the breed size.

How to More Accurately Calculate Your Pet’s Age

In 2019, a study by a research team at the University of California San Diego formulated a new way of calculating a dog’s age, based on the changes in the DNA of both dogs and humans over time. The process of using DNA methylation had already been used to study human development, through a human epigenetic clock. The 2019 study looked at the DNA sequencing and the methyl groups in a number of Labrador retrievers covering a lifespan of up to 16 years, to compare dogs’ epigenetic clocks to those of humans. The result was a new calculation (using a scientific calculator) for working out a dog’s age in human years: 16 x ln(dog’s age) + 31.

While based on a single breed, this study and a new formula have highlighted the flaws in the traditional 1:7 formula, which doesn’t take into account size, differing life stages and breed of the individual dog as dog’s age. And it has since led to the creation of the Dog Age Calendar or dog age calculator chart which is a simple tool to calculate your dog’s human age more accurately.

Common Signs of Aging in Dogs

But you don’t always need a dog age calculator to know the age of your dog in human years or where they are at in the aging cycle. There are physical signs which commonly indicate where your pet is at when it comes to getting older. And it’s these physical clues which can also help you determine your pet’s current life stage:

Their teeth

Although these will vary a little from dog to dog, as well as how well their teeth have been cared for (or not), a dog’s teeth are an excellent indicator of their life stage as a dog ages. By seven months of age, a pup will have all their permanent teeth which, by two years old, will be less white. By 5-10 years of age, there will be clear signs of wear as well as tartar build-up and by 10-15 years, their teeth will be worn, and some teeth may well be missing.

Body condition

Older dogs will tend to lose some muscle mass, meaning their skin is a little looser. They may well be stiffer in their joints and show signs of arthritis.

Grey fur

As with humans, a dog going gray is a sure sign of aging, which will start around their muzzle, before gradually spreading to other areas of the face, head, and body.

Eyes

Senior dogs are susceptible to age-related cloudiness in their eyes, as well as the formation of cataracts.

If you are in any doubt about your dog’s age, especially if they are a rescue, then a trip to the vet for a check-up will help you determine their likely age as well as any age-related health issues they may have.

Sources:

  1. Bethany Brookshire, Calculating a Dog’s Age In Human Years Is Harder Than You Think – Science News
  2. Virginia Morell, Here’s a Better Way To Convert Dog Years To Human Years, Scientists Say – American Association for the Advancement of Science
  3. How to Calculate Dog Years to Human Years – American Kennel Club
Destin Benoit
A former Special Forces Canine Handler, Destin Benoit has extensive knowledge and experience with military canine training. He has worked with multiple military dogs in the most stressful places and situations in the world. Currently, Destin is a SOC Canine Handler, aiding in the protection of the US diplomats abroad.

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