The old "multiply by seven" rule treats every dog the same, but a dog's adult size changes its aging curve more than almost anything else. Vets generally group dogs into four rough size classes for this purpose — toy/small, medium, large, and giant — and each ages on a noticeably different timeline. Here's how a few well-known breeds sort into those groups, and roughly where a 5-year-old dog of each size lands in human-year terms.
| Size class | Adult weight | Example breeds | ~5 yrs old in human years |
|---|---|---|---|
| Toy / small | Under 20 lb | Chihuahua, Pomeranian, Dachshund, Pug | ~36 |
| Medium | 21–50 lb | Beagle, Border Collie, French Bulldog | ~37.5 |
| Large | 51–90 lb | Labrador, German Shepherd, Boxer | ~37 |
| Giant | 91 lb+ | Great Dane, Mastiff, Saint Bernard | ~43 |
At five years old the size classes aren't too far apart, but the gap grows sharply from here. Giant breeds are usually considered senior by six or seven, while small breeds don't reach that stage until nine or ten — sometimes later. By age ten, a giant-breed dog is often deep into its senior years while a small-breed dog of the same age is still comfortably in its stable adult stretch.
Smaller dogs mature faster in that first year — packing roughly 15 human years into month one alone — but then settle into a long, slow plateau that can last a decade or more. Large and giant breeds mature a bit slower at the very start, but the tradeoff is a shorter overall lifespan and an earlier transition into old age, largely tied to how much tissue their bodies had to grow and maintain. We go into the biology behind that in more detail in our piece on why small dogs live longer.
Plenty of dogs — and nearly all mixed breeds — don't fall neatly into a textbook size class. If your dog's adult weight sits near a boundary (a 48-pound dog on the edge of medium and large, for instance), it's more accurate to use their exact weight rather than a breed guess. The calculator on this site supports entering an exact weight in pounds for dogs, which overrides the breed-based estimate for a more precise number.
← Try the calculator with your own dog's breed, weight, and age