Dog and cat age in human years

Nobody knows where the seven-year rule came from. Smithsonian put it plainly in 2014 — "No one knows where the dog years rule came from, though virtually all dog owners know it" — and the arithmetic that looks like its parent, seventy human years divided by ten canine ones, is a reconstruction rather than a citation. What is certain is that it is wrong in both directions at once. The chart the American Animal Hospital Association publishes alongside its 2019 Canine Life Stage Guidelines puts a one-year-old dog at fifteen human years, not seven, and a dog over 90 lb at ten years at seventy-eight where a dog under 20 lb of the same age is fifty-six. This page reads that chart, which is why it asks how heavy the dog is before it answers, and puts a second figure from the 2020 epigenetic clock beside it so you can see how far two honest methods disagree.

How it is calculated

human years = the chart row for the animal’s age and weight band

There is no equation behind the chart: it is a table, read a year at a time, with four columns for dogs at ≤20 lb, 21–50 lb, 51–90 lb and over 90 lb, and one column for cats. The four columns agree at one and two years and separate from three onwards, because dogs of different sizes age at different rates once growth is over. The second figure on this page comes from a fitted curve instead — human_age = 16 ln(dog_age) + 31, published by Wang and colleagues in Cell Systems 11 (2020) from DNA methylation in 104 Labrador retrievers — and being a curve, it has an answer everywhere the chart has none, including places the chart would rather not go.

Source: American Animal Hospital Association, "My Pet's Physiological Age" chart published with the 2019 AAHA Canine Life Stage Guidelines — dogs, physiological age in human years by weight, columns "≤20 lbs (≤9.07 kg)", "21-50 lbs (9.52-22.7 kg)", "51-90 lbs (23.1-40.8 kg)" and ">90 lbs (>40.8 kg)": 1 year 15/15/15/15, 2 years 24/24/24/24, 3 years 28/28/30/32, 4 years 32/33/35/37, 5 years 36/37/40/42, 6 years 40/42/45/49, 7 years 44/47/50/56, 8 years 48/51/55/64, 9 years 52/56/61/71, 10 years 56/60/66/78, 11 years 60/65/72/86, 12 years 64/69/77/93, 13 years 68/74/82/101, 14 years 72/78/88/108, 15 years 76/83/93/115, 16 years 80/87/99/123, 17 years 84/92/104/-, 18 years 88/96/109/-, 19 years 92/101/115/-, 20 years 96/105/120/-

Questions people ask

Where did the ×7 rule actually come from?

No source names an author. Smithsonian Magazine, reporting on the question in October 2014, wrote that "no one knows where the dog years rule came from, though virtually all dog owners know it", and noted that monks at Westminster Abbey in the 13th century had already offered a different ratio of 9:1. The usual reconstruction is that mid-century humans lived about seventy years and dogs about ten, so someone divided. That is a plausible story rather than a documented one, and this page treats it as such. What is documented is that the rule has been known to be wrong for a long time: the 2020 epigenetic clock paper checks its own curve against physiological milestones and cites work on dog ageing going back to 1953.

Why does the weight band change the answer?

Because large dogs age faster once they have finished growing, and the chart records it. All four columns read 15 at one year and 24 at two; from three years they separate, and by ten years a dog under 20 lb is at 56 while a dog over 90 lb is at 78. The 2019 AAHA Canine Life Stage Guidelines make the same point structurally rather than numerically: they define a mature adult as a dog that has completed maturation "until the last 25% of estimated lifespan (breed and size dependent)". The band is a stand-in for lifespan, and lifespan is what a human-years figure is really scaled against.

Why do the two figures disagree so badly for a young dog?

Because they are different kinds of statement. The chart is a table of physiological milestones and its first dog row is one year, where it says 15. The epigenetic clock is a logarithm, so it climbs steeply at the youngest ages and reads 31 at exactly one year. Both are published; neither is a correction of the other. The clock also comes from 104 Labrador retrievers spanning a 16-year age range, which is why it carries no weight band at all — it cannot tell you anything about a chihuahua that it does not also say about a mastiff.

Where does the chart stop, and why does it just stop?

The cat column runs to 21 years and reads 100. The dog columns run to 20 years for the three lighter bands, and the over-90-lb column simply ends at 16 years, with a dash printed in the rows below it. This page reproduces that dash rather than extrapolating past it, because a source declining to answer is information: very few dogs over 90 lb reach seventeen, so there is not much to average.

What is the chart adapted from?

The poster says so itself, in a footnote worth reading before you quote a number off it: the cat column is "adapted from AAFP-AAHA Feline Life Stage Guidelines" (Journal of Feline Medicine and Surgery, 2010, volume 12, pages 43–54), and the dog columns are "adapted from Dog Owner's Home Veterinary Handbook, Appendix B: Comparative Age of Dogs and Humans" (Wiley, 4th edition, 2007). AAHA distributes it with the 2019 Canine Life Stage Guidelines. That is a veterinary chart with a stated provenance, which is a different thing from a peer-reviewed measurement.

What do the guidelines themselves prefer to a human-years number?

Life stages. The 2019 guidelines set out puppy, young adult, mature adult, senior and end of life, and define the boundaries by proportion of expected lifespan rather than by a fixed age — senior is "the last 25% of estimated lifespan through end of life". A human-years figure is a translation for the owner; the stage is what changes what a vet does at the visit. If your animal is close to a boundary, the stage is the more useful of the two answers.

Sources

The documents this page reads its numbers out of, linked so you can check them yourself.

1. American Animal Hospital Association, "My Pet's Physiological Age" chart published with the 2019 AAHA Canine Life Stage Guidelines — dogs, physiological age in human years by weight, columns "≤20 lbs (≤9.07 kg)", "21-50 lbs (9.52-22.7 kg)", "51-90 lbs (23.1-40.8 kg)" and ">90 lbs (>40.8 kg)": 1 year 15/15/15/15, 2 years 24/24/24/24, 3 years 28/28/30/32, 4 years 32/33/35/37, 5 years 36/37/40/42, 6 years 40/42/45/49, 7 years 44/47/50/56, 8 years 48/51/55/64, 9 years 52/56/61/71, 10 years 56/60/66/78, 11 years 60/65/72/86, 12 years 64/69/77/93, 13 years 68/74/82/101, 14 years 72/78/88/108, 15 years 76/83/93/115, 16 years 80/87/99/123, 17 years 84/92/104/-, 18 years 88/96/109/-, 19 years 92/101/115/-, 20 years 96/105/120/-
2. Wang T, Ma J, Hogan AN, et al., "Quantitative Translation of Dog-to-Human Aging by Conserved Remodeling of the DNA Methylome", Cell Systems 11 (2020) — "we combined the reciprocal analyses to generate the single function: human_age = 16 ln(dog_age) + 31"
3. Rachel Nuwer, "One Human Year Does Not Equal Seven Dog Years", Smithsonian Magazine, 3 October 2014 — "No one knows where the dog years rule came from, though virtually all dog owners know it."

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