How the estimate works
The tool follows a commonly used scale. The first year of life counts as about 15 human years, since pets grow up fast, and two years is about 24. After that, each further year adds a fixed number of human years, and the amount depends on size.
Cats and small dogs under 10 kg (22 lb) add 4 years per year. Medium dogs of 10 to 25 kg (22 to 55 lb) add 5, and large dogs over 25 kg (55 lb) add 6. Larger dogs tend to age faster, which is why their number is higher.
- A 3-year-old cat or small dog: 24 + 4 = 28 human years.
- A 5-year-old medium dog: 24 + 3 x 5 = 39 human years.
- An 8-year-old large dog: 24 + 6 x 6 = 60 human years.
- A 10-year-old cat: 24 + 8 x 4 = 56 human years.
The "multiply by seven" myth
Many of us grew up hearing that one dog year equals seven human years. That is an old rule of thumb and it is not accurate. A one-year-old dog is much closer to a teenager than to a seven-year-old child, and the rate of aging slows down after the early years rather than staying constant.
Even the scale used here is an average. Breed, genetics and lifestyle all change how quickly an individual animal ages, so two dogs of the same age can be at quite different stages.
Using the number sensibly
The human-year age is a conversation starter and a way to remember that a pet in its senior years may need more rest, softer bedding and more frequent checkups. It is not a medical measurement.
For anything about your pet's diet, weight, behavior or health at a given age, ask your veterinarian, who can look at your actual animal.
Questions people ask
Why do big dogs get a higher number per year?
Large breeds tend to have shorter lifespans, so each of their years corresponds to more human years in this scale.
Is a one-year-old dog really 15 in human years?
It is an approximation. By one year a dog is physically mature or close to it, about like a teenager.
Does this work for other pets?
No. It is meant only for dogs and cats.