Dog years to human years: the chart by size, and the DNA formula

People have counted a dog's age in "dog years" for a long time. The usual rule says to multiply the dog's age by 7. The American Kennel Club chart and a 2020 DNA study both count a dog's first year as more than 7 human years, and the chart shows the pace after that depends on the dog's size.

This guide explains where the 7 year rule came from, gives the American Kennel Club chart by size, and shows the formula from a 2020 DNA study. It ends with a dog's life stages, a short section on cat years, and a simple way to record each stage with photos.

Why the "multiply by 7" rule is wrong

The American Kennel Club (AKC) says one explanation for the rule is a simple division. People lived to about 70 and dogs to about 10, so each dog year was counted as 7 human years. The AKC quotes William Fortney, a veterinarian at Kansas State University, who told the Wall Street Journal that his guess was a way to teach owners how fast dogs age, so they would bring their pets to the veterinarian at least once a year.

The problem is the first two years. The veterinary guidelines described below say a puppy's fast growth ends at 6 to 9 months, and most dogs are fully grown in body and behaviour by 3 to 4 years. A child of 7 is still years away from either. The AKC quotes the American Veterinary Medical Association (AVMA): "Contrary to popular belief, dogs do not age at a rate of 7 human years for each year in dog years."

The AVMA gives a simple guideline for a medium-sized dog:

• The first year equals 15 human years.
• The second year adds about 9 more.
• Each year after that adds about 5.

By this guideline, a 2-year-old medium dog is 24 in human years, and a 4-year-old is 34. The 7 year rule would give 14 and 28.

Dog years to human years chart by size

Size changes the count after the first few years. The AKC says larger breeds tend to have shorter lives than smaller breeds, and that research found every 4.4 pounds of body mass reduced a dog's life expectancy by about a month. It quotes the AVMA: small dogs are generally considered senior at seven years old, and larger breeds are often considered senior at 5 to 6 years of age.

The AKC chart below gives a dog's age in human years by weight. The kilogram figures in brackets are our conversion.

Dog's age	Small, 20 lb (9 kg) or less	Medium, 21 to 50 lb (10 to 23 kg)	Large, 51 to 100 lb (23 to 45 kg)	Giant, over 100 lb (45 kg)
1	15	15	15	12
2	24	24	24	22
3	28	28	28	31
4	32	32	32	38
5	36	36	36	45
6	40	42	45	49
7	44	47	50	56
8	48	51	55	64
9	52	56	61	71
10	56	60	66	79
11	60	65	72	86
12	64	69	77	93
13	68	74	82	100
14	72	78	88	107
15	76	83	93	114
16	80	87	99	121

Two things stand out. Up to age 5, small, medium and large dogs share the same numbers. After that the columns split, and by 16 a large dog is 19 human years older than a small one. Giant dogs start lower, at 12 and 22 for ages 1 and 2, then pass every other size from age 3.

The AKC reminds readers that these are estimates. It says the main data on dog lifespans comes from pet insurance companies, breed club surveys and veterinary hospitals.

The 2020 DNA study and its formula

In July 2020, a team led by Trey Ideker at the University of California San Diego published a new method in the journal Cell Systems. The team worked with the University of California, Davis, the University of Pittsburgh School of Medicine and the National Human Genome Research Institute.

The study looked at DNA methylation. Methylation marks are small chemical tags on DNA. The DNA itself stays the same through life, but the pattern of these tags changes with age. Ideker compared the tags to wrinkles on a person's face: they give a clue to age. The team read these patterns in 104 Labrador retrievers, from puppies a few weeks old to dogs of 16, according to the Cell Press release, and compared them with the same patterns in 320 people aged 1 to 103.

The result is this formula:

human age = 16 × ln(dog age) + 31

Here "ln" is the natural logarithm, a button on any scientific calculator. Enter the dog's age in years, press ln, multiply by 16 and add 31. Worked out from the formula and rounded to the nearest year:

Labrador's age	Human age from the formula
1	31
2	42
4	53
7	62
10	68
12	71
14	73

The formula counts the first years as faster than the AKC chart does, then slows down. Ideker said a one-year-old dog is "like a 30-year old human." The Cell Press release says an 8-week-old puppy matches a 9-month-old baby, and a 12-year-old Labrador matches a 70-year-old person.

The formula has one clear limit. The National Human Genome Research Institute says it is based on Labrador retrievers, and future studies with other breeds will be needed to test how well it works for them. For a small or giant breed, use the AKC chart by size.

A dog's life stages

For care and for daily life, the stage of life matters more than an exact number. The 2019 AAHA Canine Life Stage Guidelines, from the American Animal Hospital Association, divide a dog's life into five stages:

1. Puppy: from birth until fast growth stops, at 6 to 9 months, depending on breed and size.
2. Young adult: from the end of fast growth until the dog is fully grown in body and behaviour, which most dogs reach by 3 to 4 years.
3. Mature adult: from then until the last 25 percent of the dog's expected lifespan.
4. Senior: the last 25 percent of the expected lifespan, to the end of life.
5. End of life: the final stage, which depends on the dog's illness.

The senior stage moves with the breed. A breed expected to live 16 years becomes senior at 12. A breed expected to live 8 years becomes senior at 6. The guidelines suggest senior dogs have a veterinary exam at least twice a year.

What about cat years?

Cats have their own guidelines. The 2021 AAHA/AAFP Feline Life Stage Guidelines, from the American Animal Hospital Association and the American Association of Feline Practitioners, use these stages:

• Kitten: birth up to 1 year.
• Young adult: 1 to 6 years.
• Mature adult: 7 to 10 years.
• Senior: 10 years and older.
• End of life: any age.

The feline stages use fixed ages, and the guidelines give no human age equivalents.

Marking each stage with photos and portraits

A chart turns your dog's age into a number. Photos show the change itself: the oversized paws of a puppy, the full coat of a young adult, and the first grey hairs around the muzzle. Try one habit: take one photo each year, on the same date, in the same spot, at your dog's eye level. Set the photos side by side and you have your dog's life stages in order.

The puppy stage passes fastest, so take more photos in the first 9 months. Our guide on photographing puppies and kittens has tips for short attention spans. For older dogs, photographing a senior pet covers comfort, light and cloudy eyes. The general rules for a sharp, well-lit photo are in how to photograph your pet for a portrait.

Some families mark a stage with something for the wall. A royal portrait made from a clear photo of the young adult, or the senior with its grey muzzle, records the dog at that age. The gallery shows finished examples, such as a golden retriever wearing a royal crown and a red robe. The portrait studio takes one photo of up to 5 MB. If you are looking back on a dog that has died, how to memorialize a pet has ideas that start from the photos you already have.