A Dog’s Life

How do you really calculate dog years?

The oldest dog in the world, a dachshund from Long Island named Chanel, turned 21 on Wednesday. Some news reports listed her age as 120 in “dog years.”Others said she was 147. Which is it?

Chanel is about 113 dog years old. A “dog year” is a measurement that puts the age of a dog in the context of a human lifespan—in other words, if Chanel were a person, how old would she be? Most people think of one “human year” as equivalent to seven “dog years.” But that’s a bad predictor of longevity. The official formula, according to the American Veterinary Medical Association, equates the first year of a medium-sized dog’s life to 15 years of a human’s. The dog’s second year equals nine years for a human. And after that, every year feels like five for a dog. (Calculate your pet’s age in dog years here.)

This formula, however, varies depending on the dog’s weight. Bigger dogs tend to have shorter lives, and thus age faster in dog years, while smaller dogs live longer, and thus age slower in dog years. (This discrepancy is due in part to the fact that big dogs are more likely to have debilitating arthritis and stomach problems.) The life expectancy of a Great Dane, for example, is just eight years, so a 4-year-old Great Dane is already a whopping 35. That said, calculating dog years is far from an exact science, as evidenced by the fact that the AVMA’s calculator lumps all dogs more than 90 pounds—including 200-pound St. Bernards—into one category.

The formula has also changed over time, along with human and doggie life spans. Life expectancy for humans born in 1901 was 49 years. Now it’s 77. Dogs also live longer than they used to. In 1987, 32 percent of dogs lived past six years. Now about 44 percent do.

Researchers have long been intrigued by the ratio between human and canine life spans. In 1268, an inscription was etched into the floor of Westminster Abbey calculating the date of Judgment Day using the life spans of God’s creations, including the dog’s, which was considered to be nine years, and a person’s, which they said was 81 years. Eighteenth-century naturalist Georges Buffon noted that dogs lived roughly 10 to 12 years, compared with the human life span of 90 to 100 years.

It wasn’t until the 20th century that the phrase “dog years” started to appear. It’s unclear who coined it, but it was current by the 1960s, when some math textbooks had students calculate their age “in dog years.” In the 1970s, Alpo commercials featuring actor Lorne Green popularized the seven-to-one conversion: “Duchess is 13. That’s like 91 to you and me.”

The phrase “dog years” should not be confused with “dog days,” which originated in ancient times as a reference to the period in summer when the star Sirius—or the “Dog Star”—once rose with the sun. It is also distinct from the phrase “a dog’s years,” which means a very long time.

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Explainer thanks Bonnie Beaver of Texas A&M, Sharon Granskog of the American Veterinary Medical Association, and Lisa Peterson of the American Kennel Club.