Why not just multiply by 7?
The "1 pet year = 7 human years" rule is a myth. Pets mature far faster than humans in their first two years, then age more slowly afterward — so the ratio isn't constant. Veterinary guidance instead uses a curve: a steep jump in year one, another big jump in year two, then a steady yearly increase that depends on the animal's adult size.
- Year 1 — 0 → 15 human-equivalent years (rapid physical & sexual maturity)
- Year 2 — 15 → 24 (adolescence tapering off)
- Year 3+ — +4/yr (cats, small dogs) · +5/yr (medium dogs) · +5.5/yr (large dogs)
More detail
Why size changes the curve for dogs
Cats and small dogs age fairly slowly after adolescence, but large dogs' bodies work harder to sustain more mass and tend to enter their senior years sooner. That's why this calculator asks for your dog's size — a 25 kg+ dog's later years climb faster (+5.5 human years per year) than a cat's or small dog's (+4).
Where the numbers come from
The 15-then-9 pattern for the first two years comes from the AVMA's (American Veterinary Medical Association) commonly published pet age guidance. The per-year increase after year two is a separate figure: cats follow the AAFP/AAHA Feline Life Stage Guidelines (+4/year), while dogs follow a simplified linear approximation of the AKC's (American Kennel Club) size-based life-stage tables (+4 to +5.5/year depending on size) — exact figures vary by source and breed, so treat results as a helpful estimate, not a certificate.
Practical tip. If your pet is between whole years (say, 18 months), enter the decimal age (1.5) — the calculator interpolates smoothly instead of jumping between year marks.
Frequently asked questions
Is a 1-year-old dog really like a 15-year-old human?
Roughly, yes — by one year old, most dogs and cats have finished skeletal growth and reached sexual maturity, which lines up with mid-adolescence in humans. Enter age 1 for any size above to see the same 15-year result.
Why does a large dog age faster than a small dog?
After the first two years, small dogs and cats age at roughly +4 human years per year, while dogs over 25 kg are estimated at +5.5 per year in this calculator — so an 8-year-old large dog (24 + 5.5×6 = 57) reads noticeably older than an 8-year-old cat (24 + 4×6 = 48).
Why is the "multiply by 7" rule wrong?
It implies a 1-year-old dog is 7 and a 14-year-old dog is 98 — but a 1-year-old dog is already sexually mature (much closer to a human teen), and a 14-year-old dog isn't nearly 98. The real curve front-loads the aging into the first two years, which a flat ×7 multiplier can't capture.
How old in human years is my 3-year-old medium dog?
Enter 3 with "Dog — medium" selected: 24 (through year 2) + 5 × (3−2) = 29 human-equivalent years.