The 7-year rule is wrong — here is the better one
Dogs do not age at a steady seven human years per dog year. They race through puppyhood — a one-year-old dog is developmentally a teenager — and then settle into a steadier pace that depends on size, because large breeds age faster than small ones. The widely used veterinary rule this calculator follows: the first year counts as 15 human years, the second year adds 9 more (so a two-year-old is “24”), and every year after that adds 4 for small dogs, 5 for medium dogs and 6 for large dogs. Months are prorated within the current year, so a six-month-old puppy comes out at about 7½ human years.
The size rate after year two is what the flat 7× rule misses entirely: at ten years old, a small dog is about 56 in human terms, a medium dog 60, and a large dog 66 — which is why big breeds are seniors years earlier.
Worked example
A two-year-old medium dog: 15 + 9 = 24 human years, whatever its size — the size difference only starts from year three. At five years, the same medium dog is 24 + 3 × 5 = 39; a large dog the same age is 42, and a small dog 36.
How to use this calculator
- Enter your dog's age in years, plus any extra months.
- Pick the size that matches your dog's adult weight.
- Read the human-years equivalent and the life-stage label — puppy, junior, adult, mature or senior.
- Use the year-by-year table in the Detail section to see where the stage changes happen for your dog's size.
Life stages matter more than the number
The human-years figure is a conversation piece; the life-stage label is the useful part. Puppies (under 1) need socialization and frequent vet visits; juniors (1–2) are teenagers with adult bodies; adults settle until maturity, and seniors — from about 11 for small dogs, 10 for medium, 8 for large — benefit from twice-yearly check-ups, joint-friendly exercise and senior diets. For the human side of the family, the Age Calculator gives exact ages in years, months and days.