It's handy and tempting to go with the old adage that one dog year is equivalent to seven human years. However, dogs don't age at a steady rate; for example, dogs age much quicker than humans do during their first couple of years. As well, dogs age at different rates, according to size and breed.
How to Determine a Dog's Age in Human Years
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You may have two 7-year-old dogs, but if they're different sizes and breeds, they won't be the same age if you're measuring in human years. Smaller breed dogs age quicker at the beginning of their lives, reaching their adult weight earlier than large and giant breed dogs. However, after they reach adulthood, time seemingly slows down a bit for smaller dogs, as they age slower than their large counterparts. According to WebMD.com, a 7-year-old dog weighing less than 20 pounds is the equivalent of 44 years in human terms, compared to a 7-year-old dog weighing more than 50 pounds, who is the equivalent of 50 years old in the human world. If you don't have access to an age chart, you can get a rough idea of your dog's human age with a simple mathematical formula. Take your dog's age in years and subtract two; multiply the result times four and add 21. Using that formula, your 7-year-old dog is 41.