Pet age to human years calculator

The old "multiply by seven" rule is a myth. Pets do most of their growing up in a rush and then settle into a much slower pace, so Whiskerclock follows that curve instead of using a flat multiplier.

Dogs

A dog packs about 15 human years into its first year alone, then another 9 into its second. That's why a one-year-old dog is already a teenager — full-grown, and testing every boundary you set.

From the third year on, size takes over. A Great Dane and a Chihuahua can share a birthday without sharing an age: bigger dogs wear out faster, so we add roughly 4 human years per year for small breeds and up to 7 for giant ones. It's the reason a vet will treat a six-year-old Dane as a senior while a six-year-old Chihuahua is still in its prime — the canine life stage guidelines define "senior" as the last quarter of expected lifespan, and expected lifespan depends on size.

Cats

Cats front-load their ageing in much the same way. The first year is worth roughly 15 human years and the second adds about 9 more, which puts a two-year-old cat in its mid-twenties. From there it settles down to somewhere near 4 human years for every year that passes.

There's no feline equivalent of the dog study above, so these are the figures vets themselves work from — the feline life stage guidelines published jointly by the American Animal Hospital Association and the American Association of Feline Practitioners.

Indoor or outdoor makes no difference to that arithmetic. It changes the risks a cat runs, not the rate it ages — the alarming "outdoor cats live two to five years" figure you'll see quoted describes feral and stray cats, not an owned cat with a cat flap.

Why we show two numbers for dogs

The most rigorous work on dog ageing is a 2020 study from the University of California, San Diego, which read the chemical marks that accumulate on DNA over a lifetime and matched them against the same marks in humans. That's better evidence than any rule of thumb — but it profiled 104 Labrador retrievers, a single breed, so it has nothing to say about size. Its own authors call that its main limitation.

So we show both: a size-adjusted headline, and the study's figure underneath it. For a medium dog they land close together; for a Chihuahua or a Great Dane they pull apart. That gap is the honest answer, because nobody has yet published a size-aware version of the DNA work. Most calculators quietly pick one and don't mention the other.

Show the math

Dogs — size-adjusted headline

• age ≤ 1 → age × 15
• 1 < age ≤ 2 → 15 + (age − 1) × 9
• age > 2 → 24 + (age − 2) × n

n is 4 for small breeds, 5 for medium, 6 for large and 7 for giant.

Dogs — UCSD research figure

• age ≤ 1 → age × 15
• 1 < age < 2 → smooth blend between the two
• age ≥ 2 → 16 × ln(age) + 31

Cats

• age ≤ 1 → age × 15
• 1 < age ≤ 2 → 15 + (age − 1) × 9
• age > 2 → 24 + (age − 2) × 4

These are population averages. Size is accounted for above, but breed within a size band, and health, still shift the real number. Treat the result as a fun estimate, not a vet's assessment.