The formula
dogs: 16 × ln(age) + 31 · cats: 15 the first year, 9 the second and 4 after
Where it comes from
Multiplying by seven fits nothing: a one-year-old dog is already an adolescent that can reproduce, and a two-year-old is a young adult, not a child of fourteen. The real relationship is logarithmic and comes from comparing DNA methylation in dogs and people — a 2020 study that gave the formula 16·ln(age)+31 — which means ageing runs very fast at first and then slows. Size also matters enormously in dogs: a great dane ages almost twice as fast as a chihuahua, which happens in no other species.
How to work it out by hand
- Take the animal age in years, with decimals if needed
- For dogs, apply 16 times the natural log of the age, plus 31
- Adjust for size: bigger dogs age sooner
- For cats, use the stepped scale: 15, 9 and 4 years per year
What is worth knowing
The cat scale is the one from the American Association of Feline Practitioners: the first year is worth fifteen human years, the second nine and each one after that four. A two-year-old cat being twenty-four rather than fourteen is what explains why a young cat is already fully adult. The rate figure — how many human years pass per year of theirs right now — is the most useful for getting a feel: enormous in a puppy and close to four or five in an older animal.