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The honest version of the old times seven rule: fast early years, then a pace set by your dog's size.
The seven year rule is a myth that survives because it is easy. Dogs mature far faster than people in the first two years: a one year old dog is roughly a fifteen year old human in development, and a two year old is about twenty four. After that, size takes over. Small dogs age slowly and often reach their late teens; giant breeds age fast and are seniors by six. Our calculator follows the size aware method used in veterinary guidance: 15 for year one, 9 for year two, then 4 to 7 per year depending on adult size.
Last updated: November 2026. Every calculator on this site shows its method: see how we calculate and verify.
Body size changes how fast a dog wears out. Small dogs tend to live much longer, so their later years are stretched over more birthdays, while large and giant dogs reach old age sooner and pack their ageing into fewer years. That is why this calculator adds fewer human years per year for a toy dog than for a giant one once the fast first two years are over.
No, and no conversion can be. The figure is a guide built from the pattern vets use: about 15 human years for the first year of a dog's life, about 9 more for the second, then a yearly step set by adult size. Breed, health, weight and luck all move the real pace for an individual dog, so read the number as a fair picture of life stage rather than a birthday in another species.
It follows size again. Toy and small dogs are usually counted as seniors from about ten years old, medium dogs from about eight, large dogs from about seven, and giant breeds from about five or six. Our senior age calculator works through the stage for a specific breed and explains the twice yearly checkup rhythm most vets suggest from that point.