Why "7 dog years" is wrong
The rule "1 dog year = 7 human years" dates to a medieval idea that humans lived to 70 and dogs to 10 — but aging isn't linear. Puppies mature far faster than the 7× rule suggests: a 1-year-old dog is roughly a 15-year-old human, and a 2-year-old dog about 24. After that, aging slows to a steadier rate that depends on size.
The modern formula
For dogs (based on the 2019 UCSD study and AVMA guidance, adapted for size):
- First year ≈ 15 human years
- Second year adds ~9 (total 24)
- Each year after: small +4, medium +5, large +6, giant +7
For cats, the commonly used veterinary scale:
- First year ≈ 15 human years
- Second year adds ~9 (total 24)
- Each year after ≈ +4
Large dogs age faster than small ones — that's why a 10-year-old Great Dane is a senior while a 10-year-old Chihuahua is still spry. Cats are fairly consistent regardless of breed.
What this is for
Use it to time wellness visits and senior screenings (most vets recommend senior bloodwork starting around 7–8 for dogs and 10–11 for cats). It's an estimate — genetics, diet and care matter more than any formula.