My dog is 15 years old. the first two years would equate to 24 human years (15 + 9), according to this system. That leaves 13 years, each equal to about 5 human years (65). So add all this up and I get my dog is 89 human years old. Your calculator says 83. Of course, it's all approximate, but still - 6 years is a LOT different. So, what's the deal?