Shih Tzu
Dog Age Calculator
How old is your dog in human years? Not seven times their age — that myth gets puppies badly wrong and treats a Chihuahua like a Great Dane. Enter their age below and pick a size, and we will use the conversion vets actually use.
In human years
Dog years to human years
The first year of a dog’s life is worth about 15 human years, the second about nine more, and every year after that four to six depending on size. "Multiply by seven" gets a puppy badly wrong.
| Dog’s age | Small | Medium | Large | Giant |
|---|---|---|---|---|
| 1 year | 15 | 15 | 15 | 15 |
| 2 years | 24 | 24 | 24 | 24 |
| 3 years | 28 | 29 | 30 | 31 |
| 4 years | 32 | 34 | 35 | 37 |
| 5 years | 36 | 39 | 41 | 44 |
| 6 years | 40 | 44 | 46 | 50 |
| 7 years | 44 | 49 | 52 | 57 |
| 8 years | 48 | 54 | 57 | 63 |
| 10 years | 56 | 64 | 68 | 76 |
| 12 years | 64 | 74 | 79 | 89 |
| 15 years | 76 | 89 | 96 | 109 |
How the conversion actually works
The seven-year rule survives because it is easy, not because it is right, and it fails hardest at the two ends. A one-year-old dog is not a seven-year-old child: it is closer to a human teenager, able to reproduce and nearly full-grown. At the other end it flatters a Great Dane and cheats a Chihuahua, because it ignores the one factor that matters most.
The American Veterinary Medical Association publishes a chart instead, based on a medium-sized dog of 9–23 kg (21–50 lb): the first year counts as 15 human years, the second brings the total to 24, and each year after that adds roughly four to five more. The AVMA is explicit that this is approximate, and that many other factors including weight and breed influence how a dog ages.
In 2020 a team at the University of California, San Diego took a different route: rather than dividing lifespans, they read the dogs themselves. Wang and colleagues measured DNA methylation — chemical marks that accumulate on the genome in a predictable pattern with age — across 104 Labrador Retrievers spanning a 16-year age range, and compared the pattern with human methylomes. The relationship is not a straight line but a logarithmic one.
human_age = 16 × ln(dog_age) + 31
That formula makes a one-year-old dog biologically around 31, which sounds extreme until you remember such a dog can breed and has finished growing. It ages the dog fast and early, then slows down.
The three methods disagree — here is where, and by how much
Most dog-age pages publish one number and imply it is the truth. These are the three answers in circulation, side by side. The size columns are the ones this page’s calculator uses.
| Dog’s age | AVMA chart | Epigenetic clock | Small | Medium | Large | Giant |
|---|---|---|---|---|---|---|
| 1 year | 15 | 31 | 15 | 15 | 15 | 15 |
| 2 years | 24 | 42 | 24 | 24 | 24 | 24 |
| 3 years | 28 | 49 | 28 | 29 | 30 | 31 |
| 4 years | 32 | 53 | 32 | 34 | 35 | 37 |
| 5 years | 36 | 57 | 36 | 39 | 41 | 44 |
| 6 years | 42 | 60 | 40 | 44 | 46 | 50 |
| 7 years | 47 | 62 | 44 | 49 | 52 | 57 |
| 8 years | 51 | 64 | 48 | 54 | 57 | 63 |
| 10 years | 60 | 68 | 56 | 64 | 68 | 76 |
| 12 years | 69 | 71 | 64 | 74 | 79 | 89 |
| 15 years | 83 | 74 | 76 | 89 | 96 | 109 |
Read the first two columns down and something odd happens. The epigenetic clock starts far ahead — it calls a one-year-old dog 31 where the vets’ chart says 15 — and then falls steadily behind. At 13 years the two cross over, and from there the clock calls the dog younger than the chart does.
They disagree because they measure different things. The AVMA chart is a lifespan model: it maps a dog’s years onto a human’s so the proportions of a life lived roughly match. The epigenetic clock is a biological measurement of one breed’s genome, and says nothing about how long a dog will live. Neither is wrong, and the honest thing is to show both.
Two limits worth stating plainly. The methylation formula was built from Labrador Retrievers alone, so it carries no size adjustment at all — it gives a Chihuahua and a Great Dane the same answer, which is the one thing every owner knows is untrue. And it breaks down in very young puppies, where the logarithm returns a negative number.
Why large dogs age faster
Across mammal species, bigger animals live longer — an elephant outlasts a mouse by decades. Within a species the relationship inverts, and dogs are the clearest example in biology: selective breeding has produced animals differing in body mass by nearly a hundredfold, whose life expectancies differ by a factor of two.
The question is how size shortens a life, and it has a specific answer. Kraus, Pavard and Promislow fitted mortality models to death records from 74 dog breeds and separated the possibilities: do big dogs start ageing earlier, die more readily throughout life, or age at a faster rate? They found no clear link between size and the onset of senescence, and only a slightly raised baseline risk. What drove the trade-off was the rate of ageing itself — large dogs die young mainly because they age quickly.
That is why one conversion cannot serve every dog, and why this calculator asks for size before it answers. A seven-year-old Great Dane and a seven-year-old Papillon are not the same age in any sense that matters.
Practically: large and giant breeds are often considered senior at five to six, small dogs not until eight to ten. If your dog is in that band, it is worth asking your vet whether twice-yearly check-ups make sense — that is a conversation to have with them, not a conclusion to draw from a calculator.
Common questions
Is one dog year really seven human years?
No. It is a rule of thumb with no basis in veterinary medicine, and it is wrong in both directions — it badly understates a puppy’s maturity and ignores size entirely. A one-year-old dog is closer to a human teenager than to a seven-year-old child.
How old is a one-year-old dog in human years?
About 15 by the AVMA’s chart. The epigenetic-clock formula puts it closer to 31. Both agree the dog is far older than the seven-year rule suggests.
Does breed matter, or just size?
Size does most of the work, and the calculator asks for it directly. Breed matters mainly through the size it implies, though some breeds carry health conditions that affect lifespan independently. If you have a mixed-breed dog, use adult weight rather than trying to guess ancestry.
At what age is a dog senior?
It depends on size. Large and giant breeds are often considered senior at five to six years; small dogs frequently not until eight to ten. Your vet will judge it from the individual dog, not the calendar.
My dog is a puppy — why does the calculator age them so fast?
Because they genuinely are. Dogs do most of their developing in the first year, which is why that year is worth about 15 human years and the second only nine more. The curve is steep early and then flattens.
Can this tell me how long my dog will live?
No, and it deliberately does not try. It converts age; it does not predict lifespan. Anything claiming to predict how long an individual dog will live is guessing.
Sources
- American Veterinary Medical Association — “Pets age faster than people” (age conversion chart)
- Wang T, Ma J, Hogan AN, et al. “Quantitative Translation of Dog-to-Human Aging by Conserved Remodeling of the DNA Methylome.” Cell Systems 2020;11(2):176–185.e6
- Kraus C, Pavard S, Promislow DEL. “The size–life span trade-off decomposed: why large dogs die young.” The American Naturalist 2013;181(4):492–505
- American Kennel Club — “How to calculate dog years to human years”
