How to convert dog years to human years
The old rule says one dog year equals seven human years. It is easy to remember but wrong at both ends: a one-year-old dog is already a young adult, and a 10-year-old Great Dane is much older than a 10-year-old Chihuahua. Vets use a chart that does three things:
- The first year counts as about 15 human years. Dogs reach adult size and sexual maturity in their first year.
- The second year adds about 9 more, so a 2-year-old dog is roughly 24.
- After that, each year adds 4 to 7 human years depending on size. Small dogs age slowest, giant breeds fastest.
Dog age chart by size
| Dog age | Small (up to 20 lb) | Medium (21-50 lb) | Large (51-100 lb) | Giant (over 100 lb) |
|---|---|---|---|---|
| 1 | 15 | 15 | 15 | 12 |
| 2 | 24 | 24 | 24 | 22 |
| 3 | 28 | 28 | 28 | 31 |
| 4 | 32 | 32 | 32 | 38 |
| 5 | 36 | 36 | 36 | 45 |
| 6 | 40 | 42 | 45 | 49 |
| 7 | 44 | 47 | 50 | 56 |
| 8 | 48 | 51 | 55 | 64 |
| 9 | 52 | 56 | 61 | 71 |
| 10 | 56 | 60 | 66 | 79 |
| 11 | 60 | 65 | 72 | 86 |
| 12 | 64 | 69 | 77 | 93 |
| 13 | 68 | 74 | 82 | 100 |
| 14 | 72 | 78 | 88 | 107 |
| 15 | 76 | 83 | 93 | 114 |
| 16 | 80 | 87 | 99 | 121 |
The DNA formula
In 2020 researchers at UC San Diego compared age-related changes in the DNA of Labrador Retrievers and people (Wang et al., Cell Systems). Their formula is:
It says dogs age very fast early on: a 1-year-old Lab matches a 31-year-old person, a 4-year-old matches 53, and after that aging slows down. It was built from one breed, so we show it next to the size chart rather than instead of it.
Worked example
A 5-year-old medium dog, such as a Border Collie, is about 36 in human years on the size chart. The DNA formula gives 16 × ln(5) + 31 = 57. The old times-seven rule gives 35.
When is a dog a senior?
Giant breeds can be seniors at 6 or 7, while many small dogs are not senior until 10 or later. A good rule is the point where the chart passes about 56 human years. From then on, vets usually suggest checkups twice a year, blood work, and watching weight, teeth and joints.
Wondering how big your puppy will get? Try the puppy weight calculator. Have a cat too? Use the cat age calculator.
Sources: AVMA style dog age chart by size; Wang T. et al., "Quantitative Translation of Dog-to-Human Aging by Conserved Remodeling of the DNA Methylome", Cell Systems, 2020.
Frequently asked questions
How old is my dog in human years?
Count about 15 human years for the first year and 9 for the second. After that add about 4 years per year for small dogs, 5 for medium, 6 for large and 7 or more for giant breeds. A 5-year-old medium dog is about 36.
Is one dog year really seven human years?
No. Dogs grow up much faster in their first two years and then age at a rate that depends on their size. The times-seven rule is too low for young dogs and for giant breeds.
Why do big dogs age faster?
Large and giant breeds grow quickly and tend to develop age-related diseases earlier, so they have shorter lives. A Great Dane often lives 8 to 10 years while a Chihuahua can live 15 or more.
How old is a 10-year-old dog in human years?
About 56 for a small dog, 60 for a medium dog, 66 for a large dog and 79 for a giant breed.
What is the DNA formula for dog age?
Human age equals 16 times the natural logarithm of the dog's age, plus 31. It comes from a 2020 UC San Diego study of Labrador Retrievers.
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Last updated October 2026.