Dog Age Calculator: Dog Years to Human Years
How old your dog or cat is in human years, by size, not the old times-seven rule.
Good to know. A one-year-old dog is about as grown up as a fifteen-year-old human, and behaves like it.
Approximate. Real-world values vary.
How it works
In plain words
The first year counts as 15 human years and the second as 9 more, for dogs and cats. After that, add about 4 human years a year for cats and small dogs, 5 for medium dogs, 6 for large and 7 for giant breeds.
The formula
first year: 15 × age second year: 15 + 9 × (age − 1) after: 24 + years per year × (age − 2) DNA-based (dogs): 16 × ln(age) + 31
Worked example
A 5-year-old medium-sized dog: 15 for the first year, 9 for the second, and 5 for each of the next three, so 15 + 9 + 15 = 39 human years.
Sources
- American Veterinary Medical Association, Senior pets The size-based chart: 15 years, then 9, then about 4 to 7 a year.
- AAHA/AAFP Feline Life Stage Guidelines, 2021 Cat life stages: kitten, young adult, mature adult and senior from 10.
- Wang et al., Quantitative translation of dog-to-human aging by conserved remodeling of the DNA methylome, Cell Systems, 2020 The formula 16 × ln(dog age) + 31.
Sources checked September 30, 2026. 4 automated tests check its answers on every change.
About the dog age calculator: dog years to human years
The old rule of seven dog years to one human year is easy to remember, but wrong at both ends. Dogs and cats grow up fast: by their first birthday they are roughly as mature as a 15-year-old, and by two, like a person in their mid-twenties. After that, ageing slows down.
Size matters for dogs. Large and giant breeds age faster and have shorter lives than small ones: a Great Dane is a senior at six or seven, a Chihuahua may not be until ten or later. This calculator uses the approach vets use: 15 human years for the first year, 9 for the second, then about 4 to 7 a year depending on size. Cats age much like small dogs.
For dogs, it also shows an estimate from a 2020 study that compared chemical changes in the DNA of Labradors and people. It says young dogs age even faster than the charts suggest, and older dogs more slowly: a 1-year-old dog is like a 31-year-old human by that measure.
Life stages guide care: senior pets benefit from check-ups every six months, since problems like arthritis, kidney disease and dental disease are easier to treat early. Every animal is different, so treat the result as a rough guide.
Questions people ask
How old is a 5-year-old dog in human years?
About 36 for a small dog, 39 for a medium one, 42 for a large dog and 45 for a giant breed.
Is one dog year really seven human years?
No. Dogs age much faster in the first two years, about 24 human years, and more slowly after that, depending on size.
How old is my cat in human years?
A 1-year-old cat is about 15, a 2-year-old about 24, and each year after that adds about 4. So a 10-year-old cat is about 56.
When is a dog a senior?
Around the last quarter of its expected life: at about 10 for small dogs, 8 for medium, 7 for large and 6 for giant breeds.