A small dog aged 3 years 6 months is about 30 in human years
In the Japan Ministry of the Environment conversion table, a small or medium dog is 15 human years old at age 1 and 24 at age 2, then gains 4 human years for every dog year after that. Age 3 is 28 and age 4 is 32, so 3 years 6 months falls halfway between them at about 30. A large dog is 12 at age 1 and gains 7 per year after that, so the same 3 years 6 months is also about 30 (29.5), but by age 10 the two columns are far apart: 56 for small and medium dogs and 75 for large dogs.
Dogs age at different speeds depending on their size, so the age at which they become seniors differs too. Below you will find how the calculation works, the conversion table by age, the life stages, the senior age by size, and why "1 year = 7 years" is wrong.
How it is calculated
- Set the age: enter years and months, or pick a birthday. A birthday is counted in completed months up to today.
- Pick the size: choose by adult weight: small (under 20 lb / 9 kg), medium (20–50 lb / 9–23 kg), large (50–90 lb / 23–41 kg) or giant (over 90 lb / 41 kg). These are the 20, 50 and 90 lb cutoffs used by the AVMA.
- Human age: small and medium dogs use the small/medium column of the Ministry table, and large and giant dogs use the large column. When months are mixed in, the value is split proportionally between the surrounding ages.
- Life stage: set from the stage definitions in the American Animal Hospital Association (AAHA) 2019 guidelines and the senior ages by size from the AVMA.
Human age by dog age
| Dog age | Small/medium | Large | DNA methylation formula | 7x myth |
|---|---|---|---|---|
| 1 yr | 15 | 12 | 31 | 7 |
| 2 yr | 24 | 19 | 42 | 14 |
| 3 yr | 28 | 26 | 49 | 21 |
| 5 yr | 36 | 40 | 57 | 35 |
| 7 yr | 44 | 54 | 62 | 49 |
| 10 yr | 56 | 75 | 68 | 70 |
| 13 yr | 68 | 96 | 72 | 91 |
| 15 yr | 76 | 110 | 74 | 105 |
The small/medium and large columns come from the Japan Ministry of the Environment table of dog and cat ages converted to human age (values in human years). The Ministry also notes that the figures vary with breed and living conditions. The formula column is 16 ln(age) + 31, rounded, and the 7x column is included for comparison.
Life stages
| Stage | Definition (AAHA 2019) | This calculator's range |
|---|---|---|
| Puppy | From birth until fast growth stops (about 6–9 months, depending on breed and size) | Under 9 months |
| Young adult | After fast growth stops until physical and social maturity is complete (usually 3–4 years) | 9 months to 3 years; 3–4 years is the move toward mature adult |
| Mature adult | After maturity is complete until the last 25% of life expectancy | 4 years until the senior age for the size |
| Senior | From the last 25% of life expectancy to the end of life | From the senior age range for the size (within the range: entering senior) |
| End of life | The final stage, defined by illness | Not set by age |
Senior age by size
The AVMA notes that dogs differ so much in size that one senior age cannot fit every dog, and suggests treating a dog as a senior once it enters the last 25% of the life expectancy for its breed. Averages worked out from American Kennel Club (AKC) data are 8–11 years for small dogs (under 20 lb), 8–10 for medium (20–50 lb), 8–9 for large (50–90 lb) and 6–7 for giant (over 90 lb). This calculator uses those ranges to separate entering senior from senior.
| Size (study category) | Adult weight | Life expectancy at birth | Age when the last 25% starts |
|---|---|---|---|
| Toy | Under 12 lb (5.5 kg) | 13.36 yr | 10.0 yr |
| Small | 12–24 lb (5.5–11 kg) | 13.53 yr | 10.1 yr |
| Medium | 24–57 lb (11–26 kg) | 12.7 yr | 9.5 yr |
| Large | 57–99 lb (26–45 kg) | 11.51 yr | 8.6 yr |
| Giant | 99 lb (45 kg) or more | 9.51 yr | 7.1 yr |
The table above comes from a study of purebred dogs' life expectancy based on US veterinary clinic records from 2013 to 2019 (Montoya et al., 2023). The age at 0.75 times life expectancy is where the last 25% begins, and it is close to the AVMA's senior age ranges by size. Toy and small dogs are similar, but from medium size up, the larger the dog, the shorter its life expectancy, so seniority arrives earlier.
Why "1 year = 7 years" is wrong
The AVMA explains that the idea of a dog aging 7 human years for each calendar year is not true. Dogs grow very fast in the first 1 or 2 years and then age more slowly, and the pace differs with size. With the 7x rule, age 1 comes out as 7, younger than the real 15, and a 15-year-old small or medium dog comes out as 105, far above the real 76.
In 2020 researchers at UC San Diego compared DNA methylation in the blood of 104 Labrador Retrievers with that of humans and derived the formula human age = 16 ln(dog age) + 31. This formula also ages a dog quickly in the first year and more slowly later. The researchers studied only Labradors and noted that breeds with different lifespans may give different results.
Common mistakes
- Choosing the size by current weight: for a growing puppy, pick the size by the weight it will reach as an adult.
- Treating every dog as a senior from age 7: giant dogs are seniors at 6–7 and small dogs at 8–11, so it depends on size.
- Using the formula for a puppy: 16 ln(age) + 31 gives values that are too small, or even negative, under 1 year, so this calculator shows it only from age 1.
- Reading the converted age as a health status: human age is an average guide, and dogs of the same age differ by breed, weight and health.
Limits and cautions
- The Japan Ministry of the Environment table only lists whole ages from 1 to 17. Under 1 year, the result is a proportional estimate that treats age 0 as 0, and over 17 it is an estimate extended at the same step. The result is marked as an estimate.
- The Ministry table has no column for giant dogs, so giant dogs use the large column.
- Breed-specific lifespan and aging speed are not taken into account. Ask your veterinarian about senior check-ups and care.
- You can enter ages up to 30 years.
Last checked The conversion table and standards in this article were checked against the original sources of each organization and journal on 2026-10-08 (Japan Ministry of the Environment conversion table, AVMA guide to senior pets, AAHA 2019 life stage guidelines, Cell Systems 2020, Frontiers in Veterinary Science 2023).
Sources
- 01Japan Ministry of the Environment: 人間の年齢に換算した犬・猫の年齢の目安Small/medium dogs: 15 at age 1, 24 at age 2, then +4 per year; large dogs: 12 at age 1, then +7 per year (ages 1–17); varies with breed and living conditions
- 02American Veterinary Medical Association (AVMA): Caring for senior cats and dogsSenior = last 25% of life expectancy, senior age by size (AKC data), "1 year = 7 years" is not true
- 03AAHA: 2019 Canine Life Stage GuidelinesTable 1 life stage definitions (puppy, young adult, mature adult, senior, end of life)
- 04Wang T, et al. Quantitative Translation of Dog-to-Human Aging by Conserved Remodeling of the DNA Methylome. Cell Systems, 2020human_age = 16 ln(dog_age) + 31, 104 Labrador Retrievers, other breeds may differ
- 05Montoya M, et al. Life expectancy tables for dogs and cats derived from clinical data. Frontiers in Veterinary Science, 2023Size classes in Table 2; life expectancy at birth by size in the text and Supplementary Table 7 (US clinic records, purebred dogs)
Frequently asked questions
How do you convert a dog's age to human years?
In the Japan Ministry of the Environment table, a small or medium dog is 15 human years old at age 1 and 24 at age 2, then gains 4 per year. A large dog is 12 at age 1 and gains 7 per year after that. So age 5 is 36 for small and medium dogs and 40 for large dogs. When months are mixed in, the value is split proportionally between the surrounding ages, and a small dog aged 3 years 6 months is about 30.
How old is a 1-year-old dog in human years?
In the Japan Ministry of the Environment table, a small or medium dog aged 1 is 15 human years old and a large dog aged 1 is 12. In the first year the body grows fast and ages as much as a human adolescent. This calculator shows under 1 year as a proportional estimate between age 0 and age 1, so a 6-month-old small dog comes out at about 8.
Is it true that 1 dog year equals 7 human years?
No. The American Veterinary Medical Association (AVMA) says the idea that dogs age 7 human years per year is not true. With the 7x rule, age 1 comes out as 7 and age 2 as 14, younger than the real values, and a 15-year-old small or medium dog comes out as 105, far above the real 76.
Why do large dogs age faster than small dogs?
A large dog ages a little more slowly in the first year (12 at age 1) and then quickly, 7 per year. So at age 10 the Japan Ministry of the Environment table gives 56 for small and medium dogs and 75 for large dogs. A study of purebred dogs in US clinic records also found life expectancy at birth of about 13.5 years for small dogs and about 9.5 years for giant dogs, shorter the bigger the dog from medium size up.
At what age does a dog become a senior?
It depends on size. The AVMA treats the last 25% of life expectancy as senior and, using American Kennel Club (AKC) data, gives 8–11 years for small dogs (under 20 lb), 8–10 for medium (20–50 lb), 8–9 for large (50–90 lb) and 6–7 for giant (over 90 lb). This calculator shows where your dog's age falls in that range and how long is left until senior.
What is the 16 ln(age) + 31 formula?
It is a formula that UC San Diego researchers derived in 2020 by comparing DNA methylation in the blood of 104 Labrador Retrievers with that of humans. Multiply the natural log of the dog's age by 16 and add 31: age 2 gives about 42 and age 10 gives about 68. It covers a single breed and the researchers say other breeds may differ, so this calculator shows it only as a comparison.
How do I choose the size?
Choose by adult weight. Converted from the AVMA categories, small is under 9 kg (20 lb), medium is 9–23 kg (20–50 lb), large is 23–41 kg (50–90 lb) and giant is over 41 kg (90 lb). For a growing puppy, use the adult weight typical of its breed. For human age, small and medium use the small/medium column and large and giant use the large column.
What if I do not know the birthday?
Enter the age estimated in adoption records or by a veterinarian under "By age" in years and months. If you know the birthday, choose "By birthday". The age is counted in completed months as of today, and a month whose day of the month has not yet arrived is not counted.