How dog years really work
The old rule that one dog year equals seven human years is neat, memorable and wrong. Dogs do most of their growing up almost immediately: a one year old dog can already reproduce and has adult teeth, which makes it far more like a 15 year old human than a 7 year old child. The rate then slows dramatically, and from the age of two it depends heavily on how big the dog is.
The chart used above is the standard veterinary rule of thumb, in the form popularised by the American Veterinary Medical Association and the American Kennel Club. The first year counts as about 15 human years, the second adds about 9 more (so every two year old dog is roughly 24), and each year after that adds about 4 human years for a small dog, 5 for a medium dog and 6 for a large one.
Why size matters so much
In most of the animal kingdom, bigger species live longer: elephants outlast mice by decades. Within dogs the pattern flips. Small breeds routinely reach 14 to 16, while giant breeds like Great Danes average 8 to 10, and researchers still debate exactly why. The leading explanations involve faster growth in early life and the wear that comes with it; large dogs essentially age in fast-forward once they are fully grown.
That is why the chart splits by size from age two onwards, and why the size bands (under 10 kg, 10 to 25 kg, over 25 kg) matter more as a dog gets older. At age three the small and large results differ by only a couple of human years. By age ten, a small dog sits around 56 while a large dog is around 72, a gap of a whole human decade and a half.
The 2020 study, as an aside
In 2020, researchers at the University of California San Diego took a different approach. Instead of lifespans, they compared methylation, the chemical marks that accumulate on DNA as mammals age, in labradors and humans. Their curve maps a dog's age with a logarithm: human age = 16 × ln(dog years) + 31. It makes young dogs strikingly old (a two year old labrador maps to about 42) and then flattens out, so an 8 year old maps to about 64.
It is genuinely interesting science, because it measures biological ageing rather than averaging lifespans. But it was built on a single breed, and its answers for puppies feel odd for everyday use (a one year old maps to 31). For talking about your dog at the park, the classic chart above remains the sensible tool; the study is best enjoyed as evidence that dogs really do age fastest at the start.
Take it with a friendly pinch of salt
Every number on this page is an approximation. Breed matters at least as much as size, individual genetics matter more still, and diet, weight and exercise shift the picture for any dog. What the chart is good for is calibration: knowing that your 7 year old retriever is roughly a human 54 explains a lot about the greying muzzle and the longer lie-ins, and it is a helpful nudge that "senior" checkups start earlier than many owners expect, around age 7 for large breeds.
Your vet's judgement beats any formula, so treat this as conversation material rather than a health tool. Got a cat as well? The cat age calculator does the same job with the feline chart, and the age calculator handles the humans in the household. Nothing you enter here is stored or sent anywhere.
Frequently asked questions
Is one dog year really seven human years?
No, that is a myth. Dogs mature far faster than that at first: a one year old dog is roughly like a 15 year old human, not a 7 year old. The rate then slows sharply, and from age two it depends on the dog's size. The times seven rule only ever worked as a very rough average across a whole lifetime.
How does the chart actually work?
The standard veterinary chart counts the first year as about 15 human years and the second as about 9 more, so a two year old dog of any size is roughly 24. From then on each dog year adds about 4 human years for a small dog, 5 for a medium dog and 6 for a large one.
What counts as small, medium or large?
As a rough guide, small is under about 10 kg (terriers, dachshunds, most toy breeds), medium is about 10 to 25 kg (spaniels, collies, most bulldogs) and large is over 25 kg (labradors, German shepherds, and anything bigger). The boundaries are fuzzy and giant breeds age faster still, so treat the size bands as approximate.
What is the logarithmic formula I have heard about?
A 2020 study at the University of California San Diego compared chemical markers on dog and human DNA and proposed human age = 16 times the natural log of the dog's age, plus 31. It makes young dogs surprisingly old (a two year old maps to about 42) and elderly dogs converge slowly. It was based on labradors, so the classic chart remains the everyday tool.
Is this accurate for my dog?
It is a friendly approximation, not a diagnosis. Breed, weight, genetics and lifestyle all shift the picture, and no formula captures an individual dog. Your vet's assessment of body condition, teeth, joints and bloodwork says far more about your dog's biological age than any chart. Nothing you enter here is stored or sent anywhere.