People used to think that to get a rough equivalent of your dog's age in human years, you just multiplied by seven. But there was really no science behind the equation. The theory was that dogs lived about 10 years and people about 70, so divide and multiply.
But researchers have come up with a new formula to convert dog years to people years based on how canine DNA changes over time.
Geneticists at the University of California, San Diego, used a process called DNA methylation. As animals get older, DNA picks up chains of atoms called methyl groups. Because methylation happens steadily with aging, researchers can use it to estimate age in humans, according to Smithsonian. This has been called the “epigenetic clock.”
In the new paper, published in pre-print bioRxiv ahead of peer review, researchers compared the epigenetic clock in people to dogs. Although life spans in dogs vary so much by breed, they still follow a similar developmental trajectory. They reach puberty at around 10 months and die before age 20, points out Science.
For ease in research, the researchers focused on one breed, studying 104 Labrador retrievers from 4 weeks to 16 years old. They compared the canine data to methylation profiles of 320 people from age 1 to 103 years old, as well as 133 mice, reports Michelle Starr at Science Alert.
They found that people and dogs seem to age similarly when they are young, and then again in their older years.
The alignment of epigenetic clocks helped the researchers create a formula to calculate dog age in human years: human_age = 16ln(dog_age) + 31.
The "ln" is an abbreviation for the natural logarithm of your dog's age in years, which you can calculate here.
So, multiply the natural logarithm of your dog's age in years by 16, then add 31. That's your dog's age in human years.
If you don't want to do the math, Science has an embedded calculator online.
When aging syncs up
There are times when dogs and people seem to follow the same aging trajectories. A 7-week-old puppy, for example, is closely aligned to a 9-month-old human baby, both of whom are just starting to have teeth. And the formula showed that average life spans match up for Labs (12 years) and humans (70 years).
It's the adolescence and middle-age part that doesn't quite sync up. According to calculations, a 2-year-old dog is the equivalent of a 42-year-old person and a 5-year-old dog is nearly 57 in people years.
Of course, this is only one study with one breed of dog. The American Kennel Club reports that the American Veterinary Medical Association says the way to calculate people years for a medium-size dog is to count 15 years for the first year of life, nine years for the second, then about five years for each additional year.
Using this math, a 2-year-old dog would be the equivalent of a 24-year-old person and a 5-year-old dog would be 39 in people years.
Or you could just forget the math completely. After all, your dog is only as old as he feels.