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Formula
Epigenetic: Human age = 16 × ln(dog age) + 31. Size chart: 15 human years at 1, about 24 at 2, then 4 to 7 a year depending on adult size
- ln
- the natural logarithm
- dog age
- the dog's age in years, with extra months as a fraction of a year
- size
- adult weight class: small (20 lb or less), medium (21–50 lb), large (51–100 lb), giant (over 100 lb)
Two ways to turn dog years into human years
There’s no single conversion for dogs, because breeds age at very different speeds. A Chihuahua can live into its late teens, while many Great Danes are old at 8. This dog age calculator therefore gives two answers side by side.
The size chart is the one most vets and breed clubs use. It’s published by the American Kennel Club and builds on American Veterinary Medical Association guidance: about 15 human years for the first year, about 9 for the second, and then a yearly step that grows with body size.
The epigenetic formula comes from a 2020 study at UC San Diego. Researchers compared chemical marks on DNA, called methylation, between dogs and people and found that the two line up along a logarithmic curve.
How to use the dog age calculator
- Enter your dog’s age in years, plus any extra months. A puppy of 5 months is 0 years and 5 months.
- Pick the size class from your dog’s adult weight: small (20 lb or less), medium (21–50 lb), large (51–100 lb) or giant (over 100 lb). For a puppy, use the weight it’s expected to reach.
- Read the headline figure from the size chart. Underneath, you’ll see the epigenetic result, the gap between the two, and a short table comparing both methods from 1 to 16 years.
If you know the birth date rather than the age, work out the years and months first with the date difference calculator.
Dog years to human years by size
Values for whole years come from the AKC chart. The last column uses 16 ln(age) + 31, rounded.
| Dog’s age | Small | Medium | Large | Giant | Epigenetic |
|---|---|---|---|---|---|
| 1 | 15 | 15 | 15 | 12 | 31 |
| 2 | 24 | 24 | 24 | 22 | 42 |
| 4 | 32 | 32 | 32 | 38 | 53 |
| 5 | 36 | 36 | 36 | 45 | 57 |
| 8 | 48 | 51 | 55 | 64 | 64 |
| 10 | 56 | 60 | 66 | 79 | 68 |
| 12 | 64 | 69 | 77 | 93 | 71 |
| 15 | 76 | 83 | 93 | 114 | 74 |
Up to age 5 the small, medium and large columns match. The split appears at age 6. From then on small dogs add 4 human years a year, medium dogs 4 to 5 and large dogs 5 to 6. Giant breeds follow their own curve from the start, adding about 7 a year in later life.
Working it out by hand
With the size chart. Find your dog’s whole age in the table, then interpolate for extra months. A small dog of 6 years 6 months sits halfway between 40 (age 6) and 44 (age 7), so it’s about 42 in human years.
With the dog age formula. Take the natural log of the age in years, multiply by 16 and add 31. For an 8-year-old dog:
16 × ln(8) + 31 = 16 × 2.0794 + 31 = 33.27 + 31 ≈ 64
For a medium-sized dog of the same age, the chart gives 51, so the two methods differ by 13 years.
Why the two methods disagree
The epigenetic curve is steep early and flat late. A 1-year-old dog comes out at 31, reflecting how fast puppies mature biologically, and a 2-year-old at 42. After about 10, each extra year adds only one or two human years, so a 15-year-old dog is about 74.
The size chart moves the other way. It starts at a teenager’s age and keeps climbing in steady steps, which fits what owners see in old dogs, especially large breeds. The study’s authors also note that their formula was fitted on Labrador retrievers only, so they don’t claim it applies equally to every breed. Seeing both numbers is more honest than presenting one as exact.
What the 7-year rule gets wrong
Multiplying by seven gives 7 for a 1-year-old dog, yet most dogs are sexually mature by then. It also gives the same answer for a 12-year-old toy poodle and a 12-year-old mastiff, even though the mastiff is far older in biological terms. Both methods in this calculator correct those two problems in different ways.
Common mistakes
- Using today’s weight for a puppy. Size classes are based on adult weight. A 15 lb Labrador puppy belongs in the large class.
- Picking the wrong class for a mixed breed. Go by your dog’s actual adult weight, not the breeds in its ancestry.
- Reading the result as a health grade. A fit 10-year-old and a frail one get the same figure. Your vet’s examination tells you far more.
When the number is useful
Translating age into human terms helps explain why a dog of 7 may need senior bloodwork or a diet change, especially in large breeds, and why a 1-year-old still behaves like an adolescent. If you also have a cat, the cat age calculator uses a single chart with the feline life stages, since cats vary much less in size. For people, the age calculator gives an exact age in years, months and days.
Worked examples
A 3-year-old Labrador
A 3-year-old large dog (about 70 lb). The epigenetic formula puts the same dog at about 49.
Inputs: Dog’s age = 3 years; Size when fully grown = Large (51–100 lb)
Size chart, large (51–100 lb), age 3: 28 Epigenetic: 16 × ln(3) + 31 = 16 × 1.0986 + 31 = 48.6
Answer: A large dog aged 3 years is about 28 human years by the size chart and about 49 by the epigenetic formula 16 ln(age) + 31.
An 11-year-old Chihuahua
A small dog of 11; the epigenetic formula gives about 69, so the two methods differ by nine years.
Inputs: Dog’s age = 11 years; Size when fully grown = Small (20 lb or less)
Size chart, small (20 lb or less), age 11: 60 Epigenetic: 16 × ln(11) + 31 = 16 × 2.3979 + 31 = 69.4
Answer: A small dog aged 11 years is about 60 human years by the size chart and about 69 by the epigenetic formula 16 ln(age) + 31.
A 9½-year-old Great Dane
A giant-breed dog aged 9 years 6 months, interpolated halfway between the chart’s ages 9 (71) and 10 (79).
Inputs: Dog’s age = 9 years; Extra months = 6 months; Size when fully grown = Giant (over 100 lb)
Size chart: interpolate between age 9 (71) and age 10 (79) → 75 Epigenetic: 16 × ln(9.5) + 31 = 16 × 2.2513 + 31 = 67
Answer: A giant dog aged 9 years 6 months is about 75 human years by the size chart and about 67 by the epigenetic formula 16 ln(age) + 31.
A 4-month-old puppy (edge case)
A medium-breed puppy of 4 months. The chart’s first year is scaled linearly; the epigenetic formula gives about 13 because it climbs steeply in puppyhood.
Inputs: Dog’s age = 0 years; Extra months = 4 months; Size when fully grown = Medium (21–50 lb)
Size chart: 15 × 0.333 = 5 (first year scaled linearly) Epigenetic: 16 × ln(0.333) + 31 = 16 × −1.0986 + 31 = 13.4
Answer: A medium dog aged 4 months is about 5 human years by the size chart and about 13 by the epigenetic formula 16 ln(age) + 31.
Assumptions and limitations
- The size chart values for ages 1 to 16 are the AKC chart, which builds on the AVMA guideline of about 15 human years for the first year and 9 for the second. Part years are interpolated in a straight line between whole years.
- Under 1 year, the chart's first-year value is scaled in proportion to age. Past 16, the chart is extended at its last yearly step (4 for small dogs, 7 for giant dogs).
- The epigenetic formula is from Wang et al., Cell Systems (2020): human age = 16 ln(dog age) + 31, fitted on DNA methylation in 104 Labrador retrievers. It gives negative values below about 7.5 weeks, so it isn't shown for very young puppies.
- Results are rounded to the nearest whole human year. Breed and individual health aren't modeled beyond the four size classes.
- Human-year figures help picture maturity. They're an estimate, not veterinary advice.
Frequently asked questions
How do you calculate a dog's age in human years?
Count the first year as about 15 human years and the second as about 9 more. After that, add 4 to 7 human years per year depending on size, with small dogs at the low end and giant breeds at the high end. A medium-sized dog of 9 is about 56 in human years.
What is the dog age log formula?
It's the epigenetic formula human age = 16 × ln(dog age) + 31, published by UC San Diego researchers in Cell Systems in 2020. It matches changes in DNA methylation between dogs and people. For a 4-year-old dog, 16 × ln 4 + 31 = 16 × 1.386 + 31 ≈ 53.
Why do big dogs age faster than small dogs?
Large and giant breeds have shorter lifespans, so each year after the second counts for more human years. By age 10 a small dog is about 56 in human years on the chart, while a giant breed is about 79.
Is one dog year really 7 human years?
No. The 7× rule is a rough average that doesn't fit any single age well. Dogs mature very quickly at first, which the rule underestimates, and the yearly rate later depends on size.
Which method should I use, the size chart or the epigenetic formula?
The size chart is the better everyday guide because it reflects your dog's size class. The epigenetic formula reflects shared biology of aging and works well for comparing early life stages, but it was built from Labradors only and flattens out in old age.
How old is a 6-month-old puppy in human years?
On the size chart, about 7 or 8 for small to large breeds and about 6 for a giant breed. The epigenetic formula gives about 20 because it climbs steeply in the first months, when puppies develop fastest.
How do I count a dog's age if I adopted it as an adult?
Use your vet's estimate, which is based mainly on teeth, eyes and coat. Enter the estimated years and months; the result will be as good as that estimate.
Sources
- How to Calculate Dog Years to Human Years — American Kennel Club. Checked October 10, 2026.
- Quantitative Translation of Dog-to-Human Aging by Conserved Remodeling of the DNA Methylome (Wang et al., Cell Systems, 2020) — Cell Press. Checked October 10, 2026.
- Epigenetics study updates the dog-to-human age formula — National Institute on Aging (NIH). Checked October 10, 2026.