Dog Age Calculator

human age = 15 × dog age below one year; human age = 15 + 9 × (dog age − 1) between one and two years; above two years each size follows its own published table, read by linear interpolation between the rows — at four years the four figures are 32 small, 33 medium, 35 large and 37 giant

birthDate

The day the dog was born. A date in the future is rejected, because a dog that has not arrived has no age yet

breedSize

One of four weight classes: small up to 20 lb (9 kg), medium 21 to 50 lb (9 to 22 kg), large 51 to 90 lb (23 to 41 kg), giant over 90 lb (over 41 kg). These are the cut-offs printed on the chart, not a judgement about the dog

dogYears

The dog's age as a decimal number of years: days lived divided by 365.2425, the average length of a Gregorian year. This is the figure the conversion is applied to

dogMonths

Whole calendar months lived, counted from the birth date. This is the calendar reading of the age rather than the decimal one, so a dog exactly two years old reads 24 months

dogDays

Days from the birth date to today, counting the leap days in between

humanYears

The dog's age expressed in human years, rounded to one decimal place. It is an approximation drawn from a published chart, not a measurement

Use it when you want to know how old your dog is in human terms: a rescue with no paperwork, a form that asks for a pet's age, or a check on whether a dog has reached the years where the vet wants to see it more often. The breed size input is not decoration — after the second birthday it is the whole answer, and it is why a six-year-old chihuahua and a six-year-old great dane are not the same age in human terms. Owners of large and giant breeds are the ones who need this most: those dogs age fastest, and a senior dog of a large breed reaches that point at a much younger calendar age than a senior dog of a small one. The page also disposes of the old rule of thumb that one dog year equals seven human years, which understates the first two years badly and overstates the later ones.

1. A medium dog born 15 January 2022, read on 15 January 2026

   1. 15 January 2022 to 15 January 2026 is four calendar years, which is 1461 days once the leap day in 2024 is counted
   2. As a decimal age that is 1461 divided by 365.2425, which is 4.0001 years
   3. Four years is past the second birthday, so the size table applies. Medium reads 33 at four years and 37 at five, and 4.0001 of the way along that year is still 33.0
   4. In whole months the dog is 48 months old

   Four is the age where this page starts to look unlike every other one. The rule of thumb that adds five human years for every dog year after the second gives 34 here, and the published chart gives 33 — close, but the gap widens with age, and it opens in both directions depending on size.

2. A small puppy born 15 October 2025, read on 15 January 2026

   1. 15 October 2025 to 15 January 2026 is three whole calendar months, which is 92 days
   2. 92 divided by 365.2425 is 0.2518 years
   3. That is inside the first year, so the first stage applies: 15 × 0.2518 = 3.8
   4. The size makes no difference at all here — a giant puppy of the same age gives the same 3.8

   Change the size on this one and nothing moves: the first two years are shared by all four classes, which is why the chart prints a single row for them. That is worth knowing, because it means the size question only has to be answered once for dogs past their second birthday — before that, any answer will do.

3. A giant breed born 15 January 2016, read on 15 January 2026

   1. 15 January 2016 to 15 January 2026 is 3653 days, ten calendar years plus the three leap days in between
   2. As a decimal age that is 3653 divided by 365.2425, which is 10.0017 years
   3. Ten years is well past the second birthday, so the giant column of the chart applies: 78 at ten years
   4. In whole months the dog is 120 months old

   This is the case that shows what the size input is for. The same ten-year-old dog read as small comes out at 56 human years — a difference of 22 years, and enough to change the conversation at the vet. It also shows why the page reports a stage-free answer: the two dogs are the same calendar age, and no single age-based label would be right for both.

Today is taken as the UTC calendar day, so in time zones ahead of UTC the age can be a day behind the date on your wall. The conversion is a published chart rather than a measurement, and the four size classes are weight bands, not breeds: a dog that sits near a cut-off can be read either way, and the chart itself prints the bands as 20 / 50 / 90 lb, which round to 9 / 22 / 41 kg. Above the last row of the chart — 16 years for a giant breed, 20 for the other three — the figure is continued at the slope of the final two rows rather than read from the table, so the answer for a very old dog is an extrapolation. Under one year the figure is a straight line between two anchors rather than a reading from the chart, because the chart's own intermediate row does not fall on one: it puts a six-month-old at about 10 human years, where the straight line through 15 at twelve months gives about 8. This page does not show a life stage badge, and that is a decision rather than an omission: the 2019 AAHA canine life stage guidelines deliberately decline to give ages, defining mature adult and senior as the last quarter of the individual dog's estimated lifespan, which depends on breed and size. Any set of fixed ages would therefore have to be uniform across sizes — and on a page whose whole point is that size decides everything, a size-blind stage label would contradict the number printed next to it. The most widely circulated research formula for this, human age = 16 × ln(dog age) + 31, is not used here either: it comes from a study of 104 Labrador retrievers, the authors state that other breeds could give different functions, and it disagrees sharply with the chart for large dogs. And the page knows nothing about breed beyond its weight class, nor about how the dog is kept or fed, all of which move how long a dog lives.

How do I work out how old my dog is in human years?

Count the first year as about 15 human years and the second as about 9 more, so a dog is roughly 24 in human years at its second birthday. From there the weight class takes over: a small dog gains about 4 human years for each dog year, a giant breed about 7 or 8. The page does this from the birth date and the size you pick, and reports the real age in months and days as well, because a dog's calendar age in months is the figure a vet will ask for.

How old is my dog in human years if I do not know its breed?

Use its weight. The chart this page follows sorts dogs by weight rather than by breed — small up to 20 lb (9 kg), medium 21 to 50 lb (9 to 22 kg), large 51 to 90 lb (23 to 41 kg), giant over 90 lb (over 41 kg) — because weight is what correlates with how fast a dog ages, and it is the one thing you can measure without paperwork. A dog sitting exactly on a cut-off can reasonably be read either way, and the difference at ten years is a few human years.

At what age is a dog a senior?

There is no agreed number, and the veterinary guidelines decline to give one. The 2019 AAHA canine life stage guidelines define the senior stage as the last quarter of that individual dog's estimated lifespan, which depends on its breed and its size — so a great dane and a chihuahua reach it at very different ages. In practice that means a large or giant dog may be considered senior at six or seven, while a small dog may not be until ten or eleven. Ask your vet where they would put your dog, because the answer is meant to be individual.

Is one dog year really seven human years?

No, and the two ends of a dog's life are where it fails worst. The rule comes from dividing a human lifespan by a dog lifespan, which assumes a dog ages at a steady rate throughout. Dogs do not: the first year takes a dog to about 15 in human years rather than 7, and a giant breed gains about 7 or 8 human years for every year after its second birthday rather than 7 for every dog year from birth. It happens to land near the truth for a medium-sized dog in mid-life, which is why it survives.

Why do different dog age charts give different numbers?

Because several versions are in circulation and they disagree at the young end. The chart used here gives 28, 32 and 36 human years at ages three, four and five for a small dog, and separates the four sizes from three years on; the widely copied chart published under the AKC's name holds every size together at those same three figures and only parts company at age six. A third version rounds the increments to four, five, six and seven years for small through giant, which is a simplification of this table rather than the table itself — it gives 34 for a four-year-old medium dog where the chart gives 33. None of them is measuring anything; they are all rules of thumb, and the differences are largest for the largest dogs.

• Pet age poster: dog and cat age in human years, by weight class — Zoetis
• Dog age chart (small, medium, large, giant), with life stage bands — Zoetis Australia
• 2019 AAHA Canine Life Stage Guidelines — life stage checklists — American Animal Hospital Association
• How old is my dog in human years? — PetMD
• Quantitative translation of dog-to-human aging by conserved remodeling of the DNA methylome — Cell Systems (via PubMed Central)