If a dog is a year old, that is the equivalent of 15 in human years (not seven as we always thought).
When a dog reaches the age of two, he is already approximately 24 in human years (not 14 as commonly believed). But not to worry, read on.
Add four years to every year after age two. For example, a three-year-old dog is equivalent to 28 in human years; a four-year-old is 32, a five-year-old, 36, a six-year-old, 40–and so on.
Take the dog's size into consideration, since smaller dogs generally have longer life spans than larger dogs, with toy breeds tending to live the longest and giant breeds, the shortest. For example, according to the above method, a six-year-old dog is considered 40 in human years, when in fact a larger dog may actually be closer to 42. However, veterinarians consider this a good general chart to follow for all dogs.