How it works
For dogs we use the size-adjusted chart published by the American Animal Hospital Association: the first year counts as about 15 human years, the second brings the total to about 24, and later years add roughly 4 (small dogs) to 7+ (giant breeds) human years each. Ages between the chart's published rows are linearly interpolated — an approximation, clearly ours, not the chart's. The same pattern is described by the AVMA via the AKC.
Dogs one year and older also get a second opinion: the 2019 UC San Diego epigenetic-clock study fit human age = 16 × ln(dog age) + 31 to DNA methylation data. Its cohort was Labrador Retrievers only, so treat it as an interesting comparison, not gospel.
For cats we use the International Cat Care conversion: 15 human years for the first cat year, 24 by the end of the second, then 4 more for each year after. Life stages follow the 2021 AAHA/AAFP guidelines. Under one year, both species are scaled linearly toward 15 — a simplification, since puppies and kittens actually develop fastest in their first months.
Full charts: dog years to human years · cat years to human years.
FAQ
Is one dog year really seven human years?
No. The 7× rule is outdated folk math. Modern veterinary guidance (AVMA) puts a medium dog's first year at about 15 human years, the second at about 9 more, and each later year at roughly 5 — with small dogs aging slower and giant breeds faster after year two.
How do you calculate a dog's age in human years?
Count about 15 human years for the first year and 9 for the second, then add per-year amounts that depend on the dog's size — roughly 4 a year for small dogs up to 7 or more for giant breeds, following the size-adjusted chart used by the American Animal Hospital Association. See the full dog chart.
How do you calculate a cat's age in human years?
Per International Cat Care: the first year equals about 15 human years, two cat years equal about 24, and every year after that adds about 4 human years. See the full cat chart.
When is a dog or cat considered a senior?
The AVMA considers small dogs senior from about age 7 and larger breeds from 5–6. This tool labels small and medium dogs senior from 7, large and giant breeds from 6. The 2021 AAHA/AAFP feline guidelines class cats over 10 as seniors.