The old rule – multiply by seven – is easy and wrong. Dogs race through childhood: by one year most are adolescents, by two they are adults. After that, how fast they age depends on their size. This calculator gives the size rule vets use, and next to it the formula from a 2020 study of DNA changes in Labrador retrievers.
Example: a 7-year-old Border Collie, a medium dog. First year: 15 human years. Second year: +9, so 24. Then 5 a year for medium dogs: 24 + 5 × 5 = 49 human years. The DNA clock says 16 × ln(7) + 31 = 62.
Dog years to human years
| Dog’s age | Small (under 20 lb / 9 kg) | Medium (20–50 lb / 9–23 kg) | Large (over 50 lb / 23 kg) | DNA clock (2020) |
|---|---|---|---|---|
| 1 | 15 | 15 | 15 | 31 |
| 2 | 24 | 24 | 24 | 42 |
| 3 | 28 | 29 | 30 | 49 |
| 4 | 32 | 34 | 36 | 53 |
| 5 | 36 | 39 | 42 | 57 |
| 6 | 40 | 44 | 48 | 60 |
| 7 | 44 | 49 | 54 | 62 |
| 8 | 48 | 54 | 60 | 64 |
| 9 | 52 | 59 | 66 | 66 |
| 10 | 56 | 64 | 72 | 68 |
| 11 | 60 | 69 | 78 | 69 |
| 12 | 64 | 74 | 84 | 71 |
| 13 | 68 | 79 | 90 | 72 |
| 14 | 72 | 84 | 96 | 73 |
| 15 | 76 | 89 | 102 | 74 |
| 16 | 80 | 94 | 108 | 75 |
Two methods, two different curves
- The size rule (15 for the first year, 9 for the second, then 4, 5 or 6 a year) comes from a chart long used by vets in the US. It is simple and matches how small dogs outlive large ones.
- The DNA clock (human age = 16 × ln(dog age) + 31) was fitted by researchers at the University of California San Diego on the methylation of DNA in 104 Labradors, compared with people. It says a young dog ages very fast – a 1-year-old matches a 31-year-old by this measure – and an old dog slowly. It is based on one breed, and on molecular aging rather than on how a dog looks or behaves.
Both are averages. Two dogs of the same age and size can be very different: weight, exercise, teeth and genes all matter.
Life stages
The 2019 AAHA Canine Life Stage Guidelines describe five stages: puppy (from birth to the end of rapid growth, about 6–9 months), young adult (until it is physically and socially mature, by 3–4 years in most dogs), mature adult, senior (the last 25% of the expected life span, which depends on breed and size) and end of life. The boundaries are gradual, not birthdays. Each stage has its own needs: diet, dental care, joint health and how often to see the vet. The dog food calculator changes the portion for puppies and less active seniors.
Once zet4u is saved on your phone, the calculator works offline, at the vet’s or on a walk.
Frequently asked questions
Is one dog year really seven human years?
No. Dogs grow up much faster than that at the start and more slowly later. A one-year-old dog can breed and is close to adult size, more like a 15-year-old than a 7-year-old. After the second year, each year counts about 4 to 6 human years, depending on size.
How old is a 5-year-old dog in human years?
About 36 for a small dog, 39 for a medium one and 42 for a large one, by the size rule. The DNA-clock formula from a 2020 study puts it at 57: it ages dogs faster early on and slower later.
Why do big dogs age faster?
Large and giant breeds have shorter lives on average, so each year of theirs is a bigger share of a lifetime. They reach their senior years earlier, which is why the size rule adds 6 human years a year for large dogs and 4 for small ones.
When is a dog a senior?
The 2019 AAHA Canine Life Stage Guidelines call a dog senior in the last quarter of its expected life span. For a breed that usually lives 12 years, that is from about 9; for a giant breed it comes much earlier. Many vets see seniors every six months.
Sources
- Wang T. et al., Quantitative translation of dog-to-human aging by conserved remodeling of the DNA methylome, Cell Systems 11 (2020) 176–185
- AAHA: 2019 Canine Life Stage Guidelines – life stage definitions
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