Dog Years vs Human Years: What Does It Really Mean?

Where did the seven-year rule come from?

Nobody knows for certain who came up with it, but it probably rests on simple arithmetic: people used to live to around 70 on average and dogs to around 10. Divide one by the other and you get seven. The flaw is that it assumes dogs age at the same pace throughout life, and that every dog is the same size.

The first two years go fast

Dogs grow and mature very quickly. Within the first year a puppy goes from newborn to sexually mature young dog, and by two most dogs are adults both physically and mentally.

Guidance from the American Veterinary Medical Association (AVMA), as summarised by the American Kennel Club (AKC), therefore describes the first year as roughly 15 human years and the second as roughly 9 more, about 24 human years at age two. This is the guidance we use for the first two years.

After two years, size sets the pace

From the third year, ageing slows down, but not equally for every dog. Small dogs generally live longer than large ones, and large and giant breeds reach their senior years sooner.

From year three the calculator therefore uses our own size-adjusted estimation model: 4 human years per year for small dogs, 5 for medium, 6 for large and 7.5 for giant dogs. This is our simplification of the link between size and lifespan, not an official table published by the AVMA or AKC. The difference becomes clear after a few years:

What does DNA research say?

In 2020, researchers at the University of California San Diego published a study in Cell Systems comparing chemical changes to DNA (DNA methylation) in Labrador Retrievers and humans. They found that the relationship between dog age and human age follows a logarithmic curve, with the early years counting the most.

The study covered a single breed, so its formula can't simply be applied to every dog. It does confirm the main point, though: dogs age very quickly early in life and more slowly later.

How to use the numbers

• Treat human years as an estimate that makes your dog's life stage easier to understand, not as an exact age.
• Take your dog's size into account when judging whether it is approaching its senior years.
• Changes in health, weight and activity tell you more about your dog than any chart. Your veterinarian is the best source of advice for your individual dog.