A dog's first year equals about 15 human years, the second about 9 more, and each year after that about 4 to 7, depending on size — small dogs age more slowly than large ones. So a 3-year-old medium-sized dog is about 29 in human years. The old rule of multiplying by 7 is too low for young dogs and too high for old small ones.
The calculator gives the size-based figure, the newer DNA-based estimate from a 2020 study, and your dog's life stage.
Worked example
Dog's age: 3 yearsSize when grown: Medium
At 3 years old, your dog is about: 29 in human years
15 human years for the first year, 9 for the second and 5 for the third make 29. The DNA-based formula gives 16 × ln(3) + 31 ≈ 49, and the old ×7 rule just 21.
Open this example in the calculator →How to use the Dog Age Calculator
- Enter your dog's age in years, and months if you like.
- Choose your dog's size when fully grown: small, medium, large or giant.
- Read your dog's age in human years, with the DNA-based estimate and the ×7 rule for comparison.
- Check your dog's life stage and the chart for every size.
Why not just multiply by 7?
The "seven dog years for every human year" rule has no scientific basis — it comes from comparing average lifespans. Dogs grow up much faster than that: most can have puppies before their first birthday, which no 7-year-old child can. And dogs of different sizes age at different rates: a Chihuahua may live to 16, a Great Dane only to 8 or 10.
The size-based chart
Veterinary guidance, including the American Veterinary Medical Association's, puts a dog's first year at about 15 human years and its second at about 9 more. After that, a year adds roughly 4 human years for a small dog and more for bigger ones. The calculator uses 4 for small, 5 for medium, 6 for large and 7 for giant breeds.
The DNA formula
In 2020, researchers at the University of California San Diego compared changes in DNA methylation — chemical marks that build up with age — in 104 Labrador retrievers and in humans. Their formula is human age = 16 × ln(dog age) + 31. It shows a dog ageing very fast at first (a 1-year-old is like a 31-year-old) and slowly later (a 10-year-old is like a 68-year-old). It was built from one breed, so treat it as one more estimate rather than the answer.
Dog life stages
The American Animal Hospital Association's life-stage guidelines describe a puppy stage until growth stops, a young adult stage to about 3 or 4, a mature adult stage, and a senior stage covering the last quarter of the dog's expected lifespan. Because big dogs have shorter lives, they become seniors sooner: a giant breed at around 6, a small dog at around 10 or 11. Senior dogs benefit from twice-yearly checkups.
Frequently asked questions
How old is a 1-year-old dog in human years?
About 15. A dog does most of its growing up in the first year — the DNA formula puts it at 31, the age a 1-year-old dog's DNA most resembles.
Is one dog year really seven human years?
No. It is a rough average that is wrong at both ends: a 1-year-old dog is more like 15, and small dogs age more slowly than 7 years a year later in life.
How old is a 10-year-old dog in human years?
About 56 for a small dog, 64 for a medium one, 72 for a large dog and 80 for a giant breed, by the size chart. The DNA formula gives about 68.
Do big dogs age faster?
Yes. After the first two years, large and giant breeds age faster and have shorter lifespans, so they reach their senior years sooner than small dogs.
When is a dog a senior?
In its last quarter of expected lifespan — around 10 or 11 for small dogs, 8 for large dogs and 6 for giant breeds. Ask your vet about your dog.