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human-year equivalent
Dogs don't age at a constant rate: the first year takes a puppy to rough human adolescence (≈15 human years), the second adds about nine more, and only then does aging settle into a steady rate — one that depends on size. The staged method used here matches veterinary life-stage guidance far better than any single multiplier, and epigenetic research (DNA methylation clocks) confirms the fast-early, slow-later curve.
Across mammals, bigger species live longer — but within dogs, it reverses: giant breeds age fastest and live shortest, with senior status arriving around age six for a Great Dane versus eleven for a toy breed. That's why the calculator asks for size and why its later-life rate ranges from four to seven human years per dog year. Knowing your dog's stage helps time the switch to senior food and more frequent vet checks.