One human year is the equivalent of seven dog (or cat years).
Don’t believe that!
This myth presumably originated via simple math: Humans live about seven times longer than dogs. In fact, some 1960s era math textbooks asked children to calculate the corresponding “human age” of a dog using a 7:1 ratio.
The first problem with this calculation is that lifespan varies by breed. Generally, smaller dogs live longer than larger dogs. Chihuahuas, for example, average 14 – 18 years, Cocker Spaniels 12 – 15 years, and Mastiffs only 9 – 11 years. Mixed-breed dogs live about 13 years and typically enjoy better health due to their genetic diversity. Two dogs have achieved an age of 29: Max, a Terrier who passed in 2013, and Bluey, an Australian Cattle Dog who passed in 1939. This would be 203 in human years using the simple 7:1 formula (no human has lived past 122).
Secondly, dogs do not age on a linear scale. Dogs mature quickly, achieving adulthood (human equivalent of 21) at about two years old. This rate soon slows to about one dog year for every four to five human years.
There is no universally agreed-upon formula for calculating a dog’s human-equivalent age. One method is to subtract two from the dog’s age, multiply by four and then add 21. Other proposed systems give similar results, but the only accurate estimation would be from a table that considers the dog’s size/breed, as well, such as the one at http://www.petmd.com/dog/wellness/evr_dg_how_long_do_dogs_live.