Pet Age Calculator
Your dog or cat's age in human years, using the size-adjusted tables vets actually use rather than the old seven-year rule — which has never been right.
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The seven-year rule is wrong
Multiplying by seven has been the standard answer for decades, and it has been wrong the entire time. It misses two things badly. First, animals do not age at a steady rate — a dog reaches something like human adolescence within its first year, and human adulthood by two. Second, size changes everything: a Great Dane and a Jack Russell do not age at remotely the same speed.
The tables below are the ones used in veterinary practice. The first year counts for roughly fifteen human years, the second for about nine more, and every year after that adds somewhere between four and seven depending on how big the animal is.
Dogs, by size
| Size | First year | Second year | Each year after |
|---|---|---|---|
| Small, under 9 kg | 15 | +9 | +4 |
| Medium, 9–23 kg | 15 | +9 | +5 |
| Large, 23–45 kg | 14 | +8 | +6 |
| Giant, over 45 kg | 12 | +10 | +7 |
The divergence compounds. At five years old a small dog is around 36 in human terms and a giant breed is around 43. By ten, the gap has widened to roughly 56 against 78 — the difference between comfortable middle age and genuine old age.
Cats
Cats are simpler, because they vary far less in size. The first year counts for about fifteen human years, the second adds nine, and every year after that adds roughly four. A cat of twelve is somewhere around 64 in human terms, and cats reaching their early twenties is not unusual.
The epigenetic formula
In 2020 researchers at the University of California San Diego took a different approach entirely. Rather than fitting a table to life expectancy, they compared DNA methylation — chemical marks that accumulate on the genome with age — between Labrador retrievers and humans, and derived a formula from it.
It captures the shape of early ageing well: it puts a one-year-old dog at 31 and a two-year-old at around 42, which matches how quickly young dogs mature. It is shown alongside the main result for dogs over one year old. Take it as an interesting second opinion rather than the answer — it was derived from a single breed and it takes no account of size at all, so it tends to overstate the age of older small dogs.
What "senior" actually means
Senior is not a diagnosis. It is the point at which vets recommend moving from annual to twice-yearly check-ups, because the conditions that shorten life — kidney disease, arthritis, dental problems, tumours — are far more treatable when caught early. For cats that is around eleven. For dogs it depends on size: roughly ten for small breeds, eight for large, and six or seven for giants.
If the result on this page tells you your animal has crossed that line, the useful response is a check-up, not alarm.
Common questions
Is one dog year really seven human years?
No. That rule of thumb has been wrong for as long as it has been repeated. Dogs age very fast in their first two years — a one-year-old dog is roughly a fifteen-year-old human — and then much more slowly. The rate also depends heavily on size, which the seven-year rule ignores completely.
Why do big dogs age faster than small ones?
Nobody is entirely certain. The leading explanation is that large breeds grow faster and put more cellular strain on the body doing it, which brings age-related disease forward. A Great Dane is considered senior at around six; a Chihuahua not until ten or eleven.
What is the epigenetic formula?
A 2020 study from the University of California San Diego compared DNA methylation in Labradors and humans and produced the formula 16 × ln(dog age) + 31. It matches the biology well in early life but tends to overstate age in older dogs and takes no account of size, so it is shown here for comparison rather than as the main answer.
How do cats age compared with dogs?
Cats are more consistent, because size varies far less between them. The usual veterinary table puts the first year at about fifteen human years, the second at nine more, and every year after that at roughly four.
When is my pet considered senior?
For cats, around eleven. For dogs it depends on size: roughly ten to eleven for small breeds, eight for large, and six to seven for giant breeds. Senior is not an illness — it is the point at which yearly check-ups are worth making twice yearly.
Does breed matter as well as size?
It does, though size explains most of it. Some breeds are notably long-lived for their size and others carry known health problems that shorten life. Your vet knows your particular animal and its breed far better than any table can.
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Please note: CalcuLane gives estimates for general information only and is not veterinary advice. These figures are population averages and cannot account for your animal's breed, history or health. Always speak to a vet about your pet's care.