Home Blogs Need To Know with Dr Karl The truth about cat and dog years

The truth about cat and dog years

Contributor

Dr Karl Kruszelnicki

Contributor

Dr Karl Kruszelnicki

Dr Karl is a prolific broadcaster, author and Julius Sumner Miller fellow in the School of Physics at the University of Sydney. His latest book, Vital Science is published by Pan MacMillan. Follow him on Twitter at @DoctorKarl

By Dr Karl Kruszelnicki • 9 June 2025

It’s a dog’s life! Dogs age at different rates based on their size. This senior pug is in its autumn years but has outlived larger breeds such as Great Danes or Saint Bernards. Image credit: Alamy

Reading Time: < 1 • Print this page

The idea behind ‘pet years’ is to have a simple way to compare the life stages of our pets to human life spans. The standard understanding is that one pet year equals seven human years. However, the reality is much more complex. After all, most one-year-old pets can make babies – but seven-year-old humans cannot.

There are three main factors that affect the maximum life span of an animal.

First is intelligence: the ‘smarter’ the animal, the better it can adapt to its environment and survive. The second relates to environmental hazards: most animals in the wild are killed by predators, disease or injuries well before they begin losing performance as a result of ageing. And the third factor is nutrition: too much or too little food will shorten an animal’s life span.

Cats and small dogs age at a similar rate in ‘pet years’, but large dogs age much faster because they have shorter life spans. For example, a 10-year-old Great Dane will have a similar age in pet years to a 15-year-old chihuahua. Dogs age rapidly in the first two years of life, and then the rate slows down. As a general guide, a dog will age about 10–15 human years in the first year, and about nine in the second.

Once a dog turns two, each following year is equivalent to about four or five human years (small-sized dogs), five or six human years (medium-sized dogs) and six or seven years (large dog breeds).

Cats age at a similar rate to small dogs – about 15 human years in the first year of life and about nine in the second. But each following year equals four human years.

Cats and dogs have shorter life spans than humans because their organs, metabolism and cellular structures have evolved for quicker development, maturity and ageing.

However, for both cats and dogs, the simple ‘one calendar year equals seven pet years’ is definitely wrong. Many 12-year-old cats can jump tall fences and chase other cats. But you don’t see similar behaviour in most 84-year-old humans.