A dog's first year equals about 15 human years and the second brings the total to 24. After that each year adds 4 human years for a small dog, 5 for a medium dog and 6 for a large one. So a 10-year-old Chihuahua is around 56 in human terms while a 10-year-old Great Dane is around 72: same birthday, sixteen years apart in how they have aged.
Here is the full chart, the veterinary reasoning behind it, and the three places the popular shortcuts go wrong.
Dog years to human years chart
Size is measured by adult weight: small is under 22 lb, medium is 22 to 55 lb, large is over 55 lb. Enter your own numbers in the dog age calculator to skip the reading.
| Dog's age | Small (under 22 lb) | Medium (22 to 55 lb) | Large (over 55 lb) |
|---|---|---|---|
| 1 year | 15 | 15 | 15 |
| 2 years | 24 | 24 | 24 |
| 3 years | 28 | 29 | 30 |
| 4 years | 32 | 34 | 36 |
| 5 years | 36 | 39 | 42 |
| 6 years | 40 | 44 | 48 |
| 7 years | 44 | 49 | 54 |
| 8 years | 48 | 54 | 60 |
| 9 years | 52 | 59 | 66 |
| 10 years | 56 | 64 | 72 |
| 11 years | 60 | 69 | 78 |
| 12 years | 64 | 74 | 84 |
| 13 years | 68 | 79 | 90 |
| 14 years | 72 | 84 | 96 |
| 15 years | 76 | 89 | 102 |
| 16 years | 80 | 94 | 108 |
Why the first two years count for so much
A one-year-old dog is not a toddler. It is sexually mature, its skeleton has finished growing, and its adult teeth have been in place for months. Biologically that is a human somewhere in the mid teens, which is where the figure of 15 comes from.
The second year adds another 9 rather than another 15, because the growing is over and only the finishing is left: the chest fills out, the last of the adolescent behavior settles, and the dog arrives at what a human reaches around 24. From year three onward nothing dramatic is happening anymore, so the curve flattens into a steady few years per year.
This front-loading is the part every simple rule gets wrong. Aging is fast, then slow. It is not a straight line, and no single multiplier can describe it.
Why size changes the pace
Among dogs, bigger means shorter lived, which is the opposite of the pattern across species. An elephant outlives a mouse by decades, but a Great Dane rarely reaches the age an ordinary Chihuahua treats as routine.
The leading explanation is growth rate. A large breed puppy multiplies its birth weight far more, and far faster, than a small one. That pace of cell division appears to come at a cost in accumulated damage and elevated cancer risk later on. Whatever the mechanism turns out to be, the effect is consistent enough that veterinary references treat size as a required input rather than a refinement, and it is why the chart above needs three columns.
Where the multiply by 7 rule came from
It came from marketing. The ratio circulated in the mid twentieth century as a way to sell annual checkups, resting on nothing more than the rough observation that people lived to about 70 and dogs to about 10. No study produced it and none has ever supported it.
Its failure is easy to see at both ends. It puts a one-year-old dog at 7 human years, an age at which a child cannot reproduce and has years of growing left, while the dog is already an adult. Then it puts a 15-year-old small dog at 105, well past the human record, when that dog may still be climbing the stairs unassisted.
The Labrador formula, and why it is not on this page
A 2020 study in Cell Systems compared methylation patterns in dog and human DNA and produced a tidy equation: human age = 16 x ln(dog age) + 31. It is real science and it is genuinely interesting, because it is the first version of this question answered with molecular evidence rather than folklore.
The catch is the sample. The dogs were 104 Labrador Retrievers, one breed, one size class. Applied to a Yorkshire Terrier the formula returns 68 human years at age 10, which is out of step with how that dog will actually be living at 10. Until the same work is repeated across sizes, the size-based chart remains the better answer for an arbitrary dog, and that is the one the calculator uses.
When your dog becomes a senior
The American Animal Hospital Association defines the senior stage as the final quarter of a dog's expected lifespan, which is why it arrives on a different calendar for every size. In practice that means roughly 10 to 11 years for small breeds, 8 to 9 for medium and 6 to 7 for large.
The practical consequence is worth more than the number itself: senior dogs benefit from a checkup every six months instead of once a year. Twelve months is a long time in a body aging at six human years per year, and the conditions that show up in this stage, kidney disease and arthritis and dental disease among them, are far cheaper to manage when caught early. If budgeting for that is on your mind, our breakdown of what a pet actually costs per month covers where the veterinary line usually lands.
Frequently asked questions
How old is my dog in human years?
Take 15 for the first year, 24 after the second, then add 4 per year for a small dog, 5 for a medium one and 6 for a large one. A 7-year-old medium dog works out to 24 plus five lots of 5, or 49.
Is one dog year really seven human years?
No. The ratio was invented as a marketing line and no research supports it. It badly underestimates young dogs and overestimates old ones.
Do mixed breeds follow the same chart?
Yes. Use the adult weight rather than trying to identify the breeds, since weight is what the chart is built on.
Does neutering change how a dog ages?
It does not change the chart. It does lower daily calorie needs by roughly a third, and the weight gain that follows shortens lifespan on its own, so the portion size matters more after the procedure. Our guide to how many cups of dog food per day has the adjusted numbers.
What about cats?
Cats start the same way, at 15 after the first year and 24 after the second, but then add a flat 4 per year with no size adjustment. The cat age calculator handles it.