How do you calculate a dog's age in human years? In five steps, with no calculator needed: find the dog's age, find its size, use 15 for year one and 24 for year two, and then add the rate for its size — or for a cat, a flat rate — for every year after that. This guide gives the steps, worked examples at ages 1, 5, and 10, and the common mistakes. Every figure is an estimate, not an exact conversion, and the stage of life matters more than the number. The dog age calculator does the same maths for your dog's exact age and size.

The short version is: a 1-year-old dog is about 15 in human years, a 2-year-old is about 24, and after that a dog adds about 4 to 7 human years per year, depending on its size. The old rule of 'multiply by seven' is simple but wrong, and the steps below replace it. The dog age calculator guide gives the full model, and the how old is my dog in human years guide gives the worked examples.

How to calculate dog age: the steps

  • Step 1: find your dog's age in years. If you do not know it exactly, use the vet's estimate, or the age the rescue gave you — a rough age is enough for the model, because the model is an estimate.
  • Step 2: find your dog's size. Use its weight: a toy dog is typically 1 to 5 kg, a small dog 5 to 10 kg, a medium dog 10 to 25 kg, a large dog 25 to 40 kg, and a giant dog 40 to 90 kg. If you know the breed, use the breed's size from the breed data on this site.
  • Step 3: if the dog is 1 year old, the answer is about 15 human years. If it is 2 years old, the answer is about 24.
  • Step 4: if the dog is older than 2, take 24, and add the rate for its size for each year over 2: about 4 per year for a toy or small dog, about 5 for a medium dog, about 6 for a large dog, and about 7 for a giant dog. A 5-year-old medium dog is 24 plus 3 times 5, which is 39. A 10-year-old giant dog is 24 plus 8 times 7, which is 80.
  • Step 5: use the stage, not the number, to plan care. A dog that is a puppy needs puppy food and gentle exercise, an adult needs measured food and daily exercise, and a senior needs twice-yearly vet checks and gentler exercise.

Work through the steps with your own dog, and write the answer down — then check it against the table in the dog age chart by size guide or the dog age chart by breed guide. If your answer and the table agree, you have done it right. If they disagree, check the size: the most common mistake is using the wrong size, and a dog that is really medium but is calculated as large will come out too old.

Worked examples

A 1-year-old dog of any size is about 15 in human years. A 2-year-old dog of any size is about 24. A 5-year-old toy dog is 24 plus 3 times 4, which is 36; a 5-year-old medium dog is 24 plus 3 times 5, which is 39; a 5-year-old large dog is 24 plus 3 times 6, which is 42; and a 5-year-old giant dog is 24 plus 3 times 7, which is 45. A 10-year-old toy dog is 24 plus 8 times 4, which is 56; a 10-year-old medium dog is 24 plus 8 times 5, which is 64; a 10-year-old large dog is 24 plus 8 times 6, which is 72; and a 10-year-old giant dog is 24 plus 8 times 7, which is 80.

A second way to estimate a dog’s human age comes from a 2020 epigenetic study by Wang and colleagues at the University of California, San Diego, published in Cell Systems: human age equals 16 times the natural logarithm of the dog’s age, plus 31. For a 5-year-old dog that gives about 57 human years, and for a 10-year-old about 68. The two models agree best in middle age and differ most in very young and very old dogs. The dog years to human years converter gives the same figures as a converter.

Life stages for dogs

Dogs move through three broad life stages, and the boundaries depend on size:

  • Puppy or kitten (birth to about 12–18 months). Growth is rapid, especially in the first year. Feed a puppy or kitten food, feed little and often, and keep exercise gentle while joints are still developing.

  • Adult (about 18 months to senior age). Growth is finished and weight should be stable. This is the longest stage and the easiest to manage: consistent food, daily exercise suited to the breed, and a vet check once a year.

  • Senior (from about 10 years for a toy or small dog, 8 to 10 for a medium dog, 7 to 8 for a large dog, and 6 to 7 for a giant dog). The last quarter or so of a typical lifespan. Energy often drops, joints stiffen, and senses dull. Vet checks should move to twice a year, food may need adjusting, and exercise should stay regular but gentler.

Those boundaries are approximate. A dog that is lean, active, and healthy may act like an adult well past its senior boundary, while one with a health problem may need senior care earlier. The when is a dog considered a senior guide gives the thresholds by size.

Common mistakes

The most common mistake is the 'multiply by seven' rule: multiplying by seven gives 35 human years for a 5-year-old dog, against about 39 on the size model — and it is wrong in the other direction for puppies and kittens, where a 1-year-old dog is already about 15 in human years, not 7. The second mistake is using the wrong size for a dog: a dog that is really medium but is calculated as large will come out too old, and a dog that is really large but is calculated as medium will come out too young.

The third mistake is forgetting the first two years: a dog or cat does not age at a steady rate from birth, it ages fast in its first two years — 15 human years in the first year, and 24 by the end of the second — and then slows down. A calculation that starts from zero and adds a flat rate for every year will be wrong for young animals. The fourth mistake is taking the number too literally: the models give an estimate of the stage of life, not an exact conversion. The dog age in human years chart and the cat age in human years chart put every age on one page.

When the calculation is not enough

The calculation gives an estimate of the stage of life, not a diagnosis, and it is not a substitute for a vet check. A dog's weight, health, and activity matter as much as its age, and two dogs of the same age can be in very different shape. If your dog is stiff, slow, losing weight, or behaving differently, see a vet — a chart cannot tell you why, and a vet can. The dog lifespan calculator puts age in the context of lifespan, and the senior dog diet guide covers feeding in the senior years.

If you do not know your dog's age, the vet can estimate it from its teeth, its eyes, and its general condition — but the estimate is rough, and it gets rougher as the dog gets older. A rescue dog with an estimated age should be treated as an estimate, and its care should be based on its condition, not on the number. The how old is a 10-year-old dog in human years guide gives the figure for a common age, and the dog age calculator works out any age.

For related reading, see the dog age calculator, the dog age calculator guide, the how old is my dog in human years guide, the dog age chart by size guide, the dog years to human years converter, the when is a dog considered a senior guide.

Frequently asked questions

How do I calculate my dog's age in human years?

In five steps: find the dog's age, find its size, use 15 for year one and 24 for year two, and then add the rate for its size for every year after that — about 4 per year for a toy or small dog, 5 for a medium dog, 6 for a large dog, and 7 for a giant dog. A 5-year-old medium dog is 24 plus 3 times 5, which is 39. Use the table in the age chart guide to check your answer.

How old is a 1-year-old dog in human years?

About 15, whatever its size. A 1-year-old dog is already about 15 in human years, not 7 — which is why the 'multiply by seven' rule is wrong in the other direction for puppies and kittens. A 2-year-old dog is about 24.

How old is a 5-year-old dog in human years?

A 5-year-old toy or small dog is about 36, a medium dog about 39, a large dog about 42, and a giant dog about 45. Use the steps above, or the dog age calculator, for your dog's exact age and size.

How old is a 10-year-old dog in human years?

A 10-year-old toy or small dog is about 56 in human years, a medium dog about 64, a large dog about 72, and a giant dog about 80.

Is the 'multiply by 7' rule right?

No. Multiplying by seven gives 35 human years for a 5-year-old dog, against about 39 on the size model — and it is wrong in the other direction for puppies and kittens, where a 1-year-old dog is already about 15 in human years, not 7.

What if I do not know my dog's age?

The vet can estimate it from its teeth, its eyes, and its general condition — but the estimate is rough, and it gets rougher as the dog gets older. Treat an estimated age as an estimate, and base care on the dog's condition, not on the number.

Results on this page are estimates for general information only and are not veterinary advice. If your pet is ill, underweight, overweight, pregnant, or has a health condition, consult your veterinarian before changing diet or care.