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How pet age is calculated

There is no single official formula for pet age in human years. Here are the three you will see most often, what each is good for and where each falls short.

Vet chart (by size)

The chart most vets use. The first two years count for about 24 human years, then each year adds 4 to 9 more depending on size, because big dogs age faster. Cats follow one curve: 15 at one, 24 at two, then 4 per year.

It is an average. Genetics, diet and health make individual pets age faster or slower.

DNA formula (UCSD study)

From a 2020 UC San Diego study that compared age-related DNA changes in dogs and people: human age = 16 × ln(dog age) + 31. It suggests dogs age very fast when young and more slowly later.

Measured in Labrador Retrievers only, with no size adjustment, and not meaningful under one year. Dogs only.

The ×7 rule

The old rule of thumb: multiply your pet's age by seven.

A myth. It underestimates young pets (a one-year-old is closer to 15 than 7) and overestimates old small dogs.

Which one we use

Every result defaults to the size-based vet chart, interpolated by month, because it matches how vets describe aging and accounts for size. Under one year we follow a first-year curve where six months is about ten human years. Life stages follow the AAHA canine and AAHA/AAFP feline guidelines: dogs become seniors in the last quarter of their expected lifespan, cats after age ten. You can switch methods in the calculator at any time.

Try it with the dog age calculator or the cat age calculator.

Sources