Two methods, one dog
The size-based rule gives a dog 15 human years for the first year, 9 more for the second (24 in total), and then 4, 5, 6 or 7 for every extra year depending on the dog’s size. It is easy to follow and captures the fact that big dogs age faster.
The second estimate comes from a 2019 study of DNA methylation, a chemical change that tracks biological age. Its logarithmic curve climbs steeply early on and flattens later, so it gives higher numbers for young dogs and lower ones for old dogs than the rule of thumb.
A worked example
A 5-year-old medium dog: 24 for the first two years plus 3 × 5 = 15, so 39 human years by the size rule. The formula gives 16 × ln 5 + 31 = 56.8. A big gap like this is normal, and it shows these are estimates. The 7 × myth would say 35.
A 5-year-old small dog is 36 by the size rule and a large dog is 42, which is why the size selection matters.
Using the result
Life stage is more useful than the number. Puppies need vaccinations and training, adults need steady exercise and dental care, and seniors need regular health checks, joint support and diet adjustments. Your vet knows your dog and can advise on each stage.
If you also share your home with a cat, try the cat age calculator, or find your own age on other planets with the planet age calculator.
Questions people ask
How old is a 1-year-old dog in human years?
Around 15 by the size-based rule and about 31 by the 2019 genetic-clock formula. Dogs grow up very quickly in the first year or two, so a one-year-old is closer to a teenager or young adult than to a child of seven.
Is the “one dog year equals seven human years” rule true?
No. It was a rough guess based on average lifespans. Dogs mature far faster than that in the first two years and then age more slowly, and the rate depends on size. That is why this calculator offers two better estimates and shows the 7 × figure only for comparison.
How does a dog’s size change its age in human years?
Larger dogs tend to age faster and have shorter lives. After the second birthday we add 4 human years per dog year for small dogs, 5 for medium, 6 for large and 7 for giant breeds. That puts a 10-year-old small dog at 56 and a 10-year-old giant dog at 80.
What is the 2019 epigenetic formula?
Researchers at the University of California San Diego compared how DNA changes with age in dogs and humans and found human age ≈ 16 × ln(dog age) + 31. It is based on labrador retrievers, so it may fit other breeds less well, and it only applies from about a year old.
When is a dog a senior?
It varies with size. Vets commonly treat small dogs as seniors from about 10, medium dogs from 8, large from 7 and giant breeds from 6. Seniors benefit from more frequent check-ups. Ask your vet when your own dog needs them.