How to use the pet age calculator
- Choose dog or cat.
- Enter your pet's age in years and any extra months. The age must be at least 1 month and no more than 30 years.
- Read the human-equivalent age. For dogs, two published methods are shown side by side.
- Use the table to see how the methods compare at common ages. Remember that every conversion is approximate.
Formula
Dog, simple rule: 15 human years for year 1, 9 for year 2, then 5 for each year after
Dog, epigenetic formula: human age = 16 x ln(dog age) + 31
Cat rule: 15 human years for year 1, 9 for year 2 (24 total), then 4 for each year after
Partial years are prorated in a straight line inside each band of the simple rules. The epigenetic formula uses the natural logarithm, so it is not shown for dogs under 3 months.
Worked example
A 5-year-old dog gets 15 + 9 + 3 x 5 = 39 human years from the simple rule. The epigenetic formula gives 16 x ln(5) + 31 = 56.8 years. The two methods differ by almost 18 years, which shows how loose these conversions are.
A 5-year-old cat gets 24 + 3 x 4 = 36 human years.
At 1.5 years, both a dog (simple rule) and a cat come out at 19.5 human years, because 15 + 0.5 x 9 = 19.5.
Why there is no single answer
The old rule of one dog year equals seven human years is a myth. Pets do not age at a constant rate: they mature very quickly in the first two years and more slowly afterward. Dog breeds also differ in size and lifespan, and no formula here uses breed or weight.
Where the methods come from
The 15, 9 and 5 rule for dogs is a common rule of thumb that is widely repeated, including in popular pet-care references. The epigenetic formula comes from a 2020 study in the journal Cell Systems by Wang and colleagues, who compared how DNA methylation changes with age in 104 Labrador retrievers and in humans. Their fitted curve was human age = 16 ln(dog age) + 31. It was fitted to Labrador retrievers, so it may not suit very small or giant breeds.
For cats, the widely published rule is 15 years in the first year, 24 by the second and about four human years for each cat year after that, which the American Veterinary Medical Association is quoted as using.
What this is not
A human-equivalent age says nothing about your pet's health, needs or life stage. Veterinary care, diet and exercise recommendations should come from your veterinarian. If you need a date calculation for a pet's birthday or adoption day, the age calculator works from calendar dates.
Assumptions and limits
- All conversions are approximate and are for general interest. They are not veterinary advice.
- Breed, size, sex, neutering status and health are not used by any method on this page.
- The epigenetic formula was fitted to Labrador retrievers and is not defined below 3 months of age.
- Cat and dog simple rules prorate partial years in a straight line, which is a simplification.
- Ages from 1 month to 30 years are accepted. Nothing you enter leaves your browser.
Frequently asked questions
How old is my dog in human years?
It depends on the method. The simple 15, 9, 5 rule and the 2020 epigenetic formula can differ by many years, especially for older dogs. Both are shown so you can see the range instead of a single false certainty.
Is one dog year really seven human years?
No. That rule is a myth. Dogs reach maturity in their first one to two years, so early years count for much more than later ones, and the conversion is not a straight multiple.
Why is the epigenetic formula higher than the simple rule for older dogs?
The logarithm rises quickly at first and then flattens, but it starts from a higher base of 31 years at age 1. For a 10-year-old dog it gives about 67.8 years, compared with 64 from the simple rule.
How do I convert cat years to human years?
Use 15 for the first year, 24 for the second, then add about 4 for each additional year. A 10-year-old cat comes out at 24 + 8 x 4 = 56 human years.
Does this tell me my pet's life stage or health?
No. It is only an age comparison. Ask your veterinarian about the care that fits your pet's actual age, size and condition.