To convert dog years to human years, count the first year as about 15 human years, the second as about 9 more to reach 24, then add roughly 4 to 7 human years for each year after that depending on the dog’s size. The pet age calculator does this for you once you pick the species, age and size band.
Why the times-seven rule is wrong
The old idea that one dog year equals seven human years assumes a dog ages at a steady pace across its whole life. It does not. Dogs grow up fast in their first two years, reaching the rough equivalent of a young adult, and then the ageing slows. Multiplying by seven makes a puppy look far younger than it behaves and misjudges where an older dog really sits, so it is a poor guide for care.
The modern breed-size model
Vets and researchers now describe dog ageing as a curve, not a straight line:
- The first year is worth about 15 human years.
- The second year adds about 9 more, reaching 24.
- Every year after that adds a steady amount that depends on size.
Body size matters because large and giant breeds age faster and live shorter lives than small ones. That is why the calculator asks for a size band once you choose dog.
A quick guide by size
Human-years equivalent at a few common ages, by size band:
- Small (up to 20 lb): 3 years is about 28, 5 years about 36, 10 years about 56.
- Medium (21 to 50 lb): 3 years is about 29, 5 years about 39, 10 years about 64.
- Large (51 to 100 lb): 3 years is about 30, 5 years about 42, 10 years about 72.
- Giant (over 100 lb): 3 years is about 31, 5 years about 45, 10 years about 80.
The first two years read the same across sizes; the gap opens up from year three onward.
Cats age on a similar curve
Cats follow the same fast start, with the first year about 15 human years and the second reaching 24. From there each cat year adds about 4 human years, so a 10-year-old cat works out to roughly 56. Cats skip the size bands, since their range in body size is far smaller than in dogs. To work out your own age the same careful way, see the age calculator and its guide.
