Contrary to popular belief, there is no one-size-fits-all calculation when it comes to figuring your dog's age in human years. The myth that dogs age seven years for every human year is inaccurate because dogs age at different rates depending on how big or small they are.
Size is Significant
If you've lived with an array of different sized dogs, you may have already figured out that the smaller ones live longer than the larger ones. The difference in the aging process of small dogs versus big dogs is imbalanced throughout their lives, too. Smaller dogs tend to mature faster than their larger counterparts, but after the first few years things even out for awhile until the big dogs take the lead in aging at the five year mark. In her 2007 book "Caring for Your Aging Dog", Janice Borzendowski breaks dog sizes down into small, medium, large and giant to create a chart that tells readers at a glance how old their dogs are in human years. Borzendowski's table starts at five years old, at which point small dogs (20 pounds and lighter, like Chihuahuas and Yorkies) are 36, medium dogs (21 to 50 pounds, cocker spaniels, for example) are 37, large dogs (51 to 90 pounds, such as German shepherds and labrador retrievers) are 40 and giant dogs (91 pounds and bigger like great Danes and mastiffs) are considered to be 42 human years . From there each size ages at the following rates: small dogs age 4 years for every year, medium and large dogs age 5 years for every one year and giant dogs age seven years for every one year.
Smaller Dogs Live Longer, Too
Slower aging rates for small dogs means their longevity is better than larger dogs. On the average, the life expectancy for small dogs is 14 to 16 years compared to 10 to 14 years for mid-sized dogs and 10 years for large dogs. Giant dogs get the short end of the longevity stick, seldom living longer than 9 years.
Genetics of Longevity
According to the book "Dogs All-in-One for Dummies" published in 2010, breed doesn't play as large a part in the aging process as size does, although it notes that there are exceptions to that general rule because certain breeds are inclined to live lives longer or shorter than you'd expect. Examples include boxers, who tend to have the shortest life-spans of large dogs at 8 to 9 years; spaniels who, although they're mid-sized dogs, have life-spans that usually hit the far-end of the 10 to 14 year range for their size; and English bulldogs, who you would think would live to a ripe old age of 14 years being on the smaller end of the mid-sized scale, but who typically only live to the 10 year mark.
More Life Stages
There used to be only three stages of a dog's life as far as vets were concerned: puppy, adult and senior. If Fido is a medium-sized dog, he might get his senior citizen's card when he reaches the age of eight, meaning he's old enough to eat senior-formula food and start receiving senior care from his vet. But with advancements in pet healthcare, just like humans, dogs are living longer lives. This has caused the veterinary community to add geriatrics as a fourth life-stage to their wellness programs. This has skewed the numbers a bit, causing your mid-sized Fido as well as small dogs to be considered seniors at age seven and hitting geriatric status when they reach age 13. As for large dogs, they're considered seniors at 6 years old while giant dogs are seniors at 5 and both are geriatric at 10.
- Web MD: How to Calculate Your Dog's Age
- Caring for Your Aging Dog: A Quality-Of-Life Guide for Your Dog's Senior Years; Janice Borzendowski
- Dogs All-In-One for Dummies; Consumer Dummies
- Brand X Pictures/Brand X Pictures/Getty Images