Why are dog years calculated?

How old is your tail-wagging bundle of joy in human years? According to the well-known "rule of paw," one dog year is the equivalent of 7 years. Now, in a study published July 2, in the journal Cell Systems, scientists say it's wrong. Dogs are much older than we think, and researchers devised a more accurate formula to calculate a dog's age based on the chemical changes in the DNA as organisms grow old.

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The More Accurate Calculation of Dog Years
Dog Age Calculator: Dog Years to Human Years
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Dogs share the same environment as their owners and receive almost the same standard of health care as humans, providing a unique opportunity for scientists to understand aging across species. Like humans, dogs follow similar developmental trajectories that lead them to grey and become more susceptible to age-related diseases over time. However, how they age on a molecular level is more complicated -- aging rapidly at first and slowing down later in life.

"In terms of how physiologically mature a 1-year-old dog is, a 9-month-old dog can have puppies. Right away, you know that if you do the math, you don't just times seven," says senior author Trey Ideker (@TreyIdeker) of the University of California, San Diego. "What's surprising is exactly how old that one-year-old dog is -- it's like a 30-year old human."

Human and dog DNA, which codes who we are, doesn't change much throughout the course of life, but chemical marks on the DNA, called methylation marks, do. Ideker considers these marks like wrinkles in the genome. "I tend to think of it very much like when you look at someone's face and guess their age based on their wrinkles, gray hair, and other features," he says. "These are just similar kinds of features on the molecular level."

The researchers studied 104 Labrador retrievers spanning from few-week-old puppies to 16-year-old dogs with the help of two canine experts, Danika Bannasch of the University of California, Davis, and Elaine Ostrander of the National Institutes of Health. They compared the changes in the methylation pattern to humans.

The comparison revealed a new formula that better matches the canine-human life stages: human age = 16 ln(dog age) + 31. Based on the new function, an 8-week-old dog is approximately the age of a 9-month-old baby, both being in the infant stage where puppies and babies develop teeth. The average 12-year lifespan of Labrador retrievers also corresponds to the worldwide life expectancy of humans, 70 years.

"I like to take my dogs on runs, and so I'm a little bit more sympathetic to the 6-year-old now," says Ideker, who realized that his dog is pushing 60 according to the new calculation.

In both species, they found that the age-driven methylation largely happens in developmental genes that are hotly fired up to create body plans in utero and regulating childhood development. By the time one becomes an adult and stops growing, "you've largely shut off these genes, but they're still smoldering," says Ideker. "If you look at the methylation marks on those developmental genes, they're still changing."

Focusing on the smoldering developmental genes, the team developed a clock that can measure age and physiological states across different species, while other methylation-quantifying age-predicting methods only do well in one species. Ideker also noted that future investigation in different dog breeds with various lifespans could provide more insight into the new clock. The clock may not only serve as a tool to understand cross-species aging but also apply as clinical practice for veterinarians to take proactive steps to treat animals.

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The study involved 104 Labrador retrievers between four weeks and 16 years old. Herwig Kavallar via Wikimedia Commons under Public Domain

One dog year is not equivalent to seven human years, despite widespread use of the ratio for calculating the age of canine companions. Presumably, the ratio is based on the average lifespan of dogs being 10 years and humans being 70 years, it’s not quite so simple. The formula is not based on any real science and it was debunked by veterinarians years ago.

But geneticists digging into the mysteries of aging have developed a new calculation to understand how our canine companions' ages correspond to our own.

To understand how dogs age, the team looked at a phenomenon called DNA methylation. As mammals get older, their DNA picks up methyl groups that "stick" to their DNA. While these groups don’t change the DNA itself, they attach to the genetic molecule and can turn certain genes on or off, which is an important part of epigenetics, or the way environmental factors cause certain genes to express themselves.

Methlyation occurs at a relatively steady rate as humans age, which allows researchers to estimate a person's age, a process they’ve dubbed the “epigenetic clock.”

In the new paper on dog years, which has yet to be peer reviewed and is currently posted on the preprint server bioRxiv, a team led by Tina Wang of the University of California, San Diego, compared the epigenetic clocks in people to canines to better understand the genes associated with aging. They picked dogs because most live in the same environments as humans and also receive some degree of medical care, like humans do.

The team looked at methylation rates in 104 Labrador retrievers between the ages of four weeks and 16 years old, reports Michelle Starr at Science Alert. They then compared them to published methylation profiles of 320 humans from age one to 103. (They also compared both to 133 mice methylation profiles.)

It turns out some parts of a dog’s life follows the same pattern as humans, though other longevity milestones don’t link up quite as nicely. For instance, the methylation rate showed a seven-week-old pup corresponds to a 9-month-old human baby, and both species begin to get their first teeth at this time.

But the comparison breaks down after early puppyhood. The dog clock ticks much faster with pups speeding through puberty and reaching sexual maturity within their first year. Then, the dog's epigenetic clock slows down as the dog ages, and begins to match up with humans again in its later years.

Overall, the average 12-year lifespan of a Labrador lined up with the average worldwide lifespan of humans, which is about 70 years.

While the study complicates the concept of “dog years,” it does show that the animals experience similar methylation processes as humans.

“We already knew that dogs get the same diseases and functional declines of aging that humans do, and this work provides evidence that similar molecular changes are also occurring during aging,” Matt Kaeberlein, a biogerontologist at the University of Washington who was not involved in the study, tells Virginia Morell at Science. “It’s a beautiful demonstration of the conserved features of the epigenetic age clocks shared by dogs and humans.”

The new formula for a dog’s ages based on the study requires a little more math than multiplying by seven. You multiply the natural logarithm of a dog's age by 16, then add 31 [human_age = 16ln(dog_age) + 31].

According to the formula, a 2-year-old dog is the equivalent of a 42-year-old human, but things slow down after that. A 5-year-old dog is the equivalent of a 56.75 year old human, and a 10-year-old dog is the equivalent of 67.8-year-old person.

Evolutionary biologist Steve Austad of the University of Alabama in Birmingham, who was not involved in the study, tells Morell that he’s not too surprised that the epigenetic clock applies to dogs, too. He says that by studying different dog breeds with different lifespans the researchers may find some interesting results.

This formula is not the last word on dog years, however, especially since it only looked at one breed. Erika Mansourian, writing for the American Kennel Club, reports that the American Veterinary Medical Association says the accurate way to calculate dog years for a medium-sized dog is to assume the first year is equivalent to 15 years and age two adds another nine years. After that, each year of a dog’s life is equivalent to five human years. It doesn't perfectly line up with the new formula, but both acknowledge that dogs age rapidly in their first years of life.

Whatever the case, dogs’ lives are all too short. That may be why people are excited about a project by the Dog Aging Project, which is currently recruiting 10,000 pets and their owners to participate in a new study that will look at the dogs' health, gut microbes, diet and exercise to understand aging. And 500 lucky dogs will test out a new drug that may help slow the aging process, which could help us someday, too.

How old is your dog in human years? We used to simply multiply a dog’s age by seven. However, this calculation isn’t so cut-and-dried. See our dog age calculator and chart to discover your dog’s age.

The easy way to calculate a dog’s age is to take 1 dog year and multiple it by 7 years. This is based on an assumption that dogs live to about 10 and humans live to about 70, on average.

For example, a dog which is 5 years old is 35 “human years.” This isn’t a bad way to go from a health perspective because it helps us humans realize that our dogs aren’t the same as children.

As pets get older, they need extra care and attention. Small dogs are generally considered “senior” at seven years of age. Larger breeds are often senior when they are 5 to 6 years of age.

Be aware of arthritis and related discomfort or irritability, weight control, sight and hearing issues, and any changes in behavior or activity which could indicate more serious issues.

Visit your vet for exams regularly; adjustments can be made to give your pets a more comfortable, longer, healthier life.

The More Accurate Calculation of Dog Years

Statistics from pet-insurance companies, breed-club surveys, and veterinary hospitals have helped us learn more about how dogs age.

Contrary to common belief, dogs do not age at a rate of 7 human years for each year in dog years.

According to the American Veterinary Medical Association:

The first year of a medium-sized dog’s life is equal to approximately 15 years of a human’s life.
The second year of a dog’s life equals about nine years for a human.
And after that, every human year equals approximately four or five years for a dog.

In other words, the number of human years piles onto a dog’s life more quickly during the dog’s rapid growth to maturity.

Dog Age Calculator: Dog Years to Human Years

If we think like a dog, here’s how a dog’s age compares to a human’s age! Note that this calculator is for a medium size breed of dog. Consult the chart below to see the differences between different size dogs (small, medium, large, giant). It should give you a good sense of where your dog is in the development/aging process.

Age of Dog (dog’s age according to the calendar)	Dog’s Age in Human Years (dog’s age in equivalent human years, based on stage of breed size)
Small	Medium	Large	Giant
1 year	15	15	15	12
2 years	24	24	24	22
3	28	28	28	31
4	32	32	32	38
5	36	36	36	45
6	40	42	45	49
7	44	47	50	56
8	48	51	55	64
9	52	56	61	71
10	56	60	66	79
11	60	65	72	86
12	64	69	77	93
13	68	74	82	100
14	72	78	88	107
15	76	83	93	114
16	80	87	99	121