Forget the "×7" myth — this uses the more accurate, non-linear aging curves vets actually reference.
Choose dog or cat, enter the pet's age in years, and for a dog pick a size band. The page returns a human-age equivalent and tells you which curve it used. Fractions count: a six-month-old puppy is 0.5 years and should be entered that way.
The size field only appears for dogs, because it genuinely changes the answer. A Chihuahua and a Great Dane are the same age in years and not at all the same age in body.
A three-year-old dog, whatever its size:
| Method | Result |
|---|---|
| Multiply by seven | 21 |
| Logarithmic curve, 16 × ln(age) + 31 | about 49 |
Forty-nine is the better answer, and the reason is developmental. A three-year-old dog is a mature adult: it has finished growing, its teeth are set, and its behaviour has settled. A 21-year-old human is still in the middle of becoming an adult. The multiply-by-seven rule was never a biological claim; it was a convenience that happens to be roughly right in the middle of a dog's life and wrong at both ends.
For a cat, the convention is steeper early and flatter later: 15 human years for the first year, another 9 for the second, and about 4 for each year after. A three-year-old cat lands at 28.
The logarithmic formula used for dogs here comes from a 2020 study by Wang and colleagues in the journal Cell Systems, which compared chemical marks on DNA in dogs and humans and found that ageing slows in dogs relative to calendar time as they get older. That is why the curve is steep at the start and flatter later rather than a straight line.
Size is the second variable and the bigger one after middle age. Large dogs age faster in the later years, which is why the page adds an extra allowance for big breeds past seven and why a Great Dane at ten is genuinely old while a small terrier at ten is often still spry. Lifespan follows the same pattern: small breeds commonly reach the mid-teens, giant breeds rarely do.
The number is a talking point and a rough guide to life stage, not a clinical measure. Your vet assesses age from teeth, eyes, muscle and bloodwork, which is far more informative than a single converted figure.
It assumes a pet ages at a constant rate. In reality the first two years cover most of the maturation, and the later years pass more slowly in relative terms.
Yes for large breeds in later life. They mature more slowly as puppies and age faster as seniors, which is why the page adds an allowance after age seven.
As a decimal. Six months is 0.5, and eighteen months is 1.5.
No. Vets assess life stage from physical examination. This is an estimate for conversation, not a clinical tool.
Because the first two years of a cat's life cover the equivalent of about 24 human years of development.
The dog equation is from a 2020 paper on Labrador retrievers. The cat steps and the large-dog adjustment are not in that paper.
Last updated: November 2, 2026. Reviewed by David Liu.