Every year we hear the Alpo man claim his 14-year-old dog is actually 98 years old in human years. Yet I remember reading in some journal that the seven-years-for-every-one-dog-year rule is only applicable to dogs under 10 years old. After a certain age the conversion factor reduces considerably, so that a 98-year-old Alpo dog is really in his mid-70s. I need the Straight Dope so I can get the candles right for my hound.
— Michael L., Bethesda, Maryland
I've seen various formulations for this over the years. One of the simplest and most sensible goes like this: The first year of canine life is equal to 21 years of human life — in other words, the puppy grows to adulthood. Every additional dog year is equivalent to four human years. Thus a 10-year-old mutt is the equivalent of 57 human years old (9 x 4 + 21). Likewise, the Alpo dog is not 98 (14 x 7) but 73 in human terms (13 x 4 + 21).
The formula jibes reasonably well with the known landmarks of canine life. Dogs reach middle age when they're 6 or 7, which works out to 41-45 in HY. Life expectancy for most is 12-15 years (65-77 HY); occasionally one manages to creak along until age 20 (97 HY). That makes a lot more sense than saying a 20-year-old dog is 140, and it sure saves on the candles, too.
— Cecil Adams