How the calculator works
The calculator shows two methods side by side, because they answer the question differently.
1. The size chart. Veterinary sources commonly publish a chart in which every dog counts 15 human years at its first birthday and 24 at its second, after which bigger dogs age faster. The calculator turns that chart into one simple rule per size, based on adult weight:
| Size | Adult weight | Each year after two adds |
|---|---|---|
| Small | 20 lb or less (up to 9 kg) | 4 human years |
| Medium | 21–50 lb (10–23 kg) | 4.5 human years |
| Large | 51–90 lb (23–41 kg) | 5.5 human years |
| Giant | over 90 lb (over 41 kg) | 7 human years |
Under a year it scales in a straight line from 0 to 15, so a 6-month-old puppy comes out at about 7.5. Puppies develop in spurts, so treat puppy numbers loosely.
2. The epigenetic formula. Researchers at UC San Diego led by Tina Wang compared age-related changes in DNA methylation, chemical marks on DNA that shift as we age, in Labrador retrievers and in people. Their study, shared as a preprint in 2019 and published in Cell Systems in 2020, proposed:
human age = 16 × ln(dog age) + 31, where ln is the natural logarithm.
It says dogs age very quickly at first and slowly later: a 1-year-old Lab works out to 31, a 10-year-old to about 68. Keep two limits in mind. It was derived from Labradors only, so it has not been shown to fit other breeds or sizes. And it is not meant for young puppies; for the youngest it even gives negative ages. The calculator shows it only for dogs a year and older.
Why not just multiply by seven?
Nobody is quite sure where the seven-to-one rule came from; it may simply compare rough human and dog lifespans. It fails at both ends. A 1-year-old dog is nearly adult, not a 7-year-old child, and the rule ignores size: by 16, a small dog is about 80 on the size chart, while a giant-breed dog, if it got that far, would be about 122.
Dog age chart by size
Human ages are rounded to the nearest year.
| Dog's age | Small | Medium | Large | Giant | Epigenetic (Labrador) |
|---|---|---|---|---|---|
| 1 | 15 | 15 | 15 | 15 | 31 |
| 2 | 24 | 24 | 24 | 24 | 42 |
| 3 | 28 | 29 | 30 | 31 | 49 |
| 4 | 32 | 33 | 35 | 38 | 53 |
| 5 | 36 | 38 | 41 | 45 | 57 |
| 6 | 40 | 42 | 46 | 52 | 60 |
| 7 | 44 | 47 | 52 | 59 | 62 |
| 8 | 48 | 51 | 57 | 66 | 64 |
| 9 | 52 | 56 | 63 | 73 | 66 |
| 10 | 56 | 60 | 68 | 80 | 68 |
| 11 | 60 | 65 | 74 | 87 | 69 |
| 12 | 64 | 69 | 79 | 94 | 71 |
| 13 | 68 | 74 | 85 | 101 | 72 |
| 14 | 72 | 78 | 90 | 108 | 73 |
| 15 | 76 | 83 | 96 | 115 | 74 |
| 16 | 80 | 87 | 101 | 122 | 75 |
The giant column past about 10 years is mostly theoretical, since few giant breeds live that long.
Example: a 7-year-old Labrador
A 7-year-old Lab that weighs 65 lb is a large dog.
- Size chart: 24 + 5 years × 5.5 = 51.5, so about 52.
- Epigenetic formula: 16 × ln(7) + 31 = 16 × 1.946 + 31 = 62.1, so about 62.
- Times seven: 49.
The two methods are ten years apart for the very breed the formula was built on. That is a good reminder that "dog years" describe a stage of life, not a measurement.
Tips
- Go by adult weight. For a puppy or a mixed breed, use the weight your dog is expected to reach as an adult.
- Don't use dog years for doses or food. Medication and feeding amounts depend on weight and your veterinarian's advice, not on a human-age equivalent.
- Aging varies. Breed, genes, weight and health all play a part, and two dogs of the same age and size can age very differently.
- Watch for changes. Stiffness, weight change, drinking more or new behavior is worth a vet visit at any age.
Frequently asked questions
How do I calculate my dog's age in human years?
Count 15 human years for the first year and 9 more for the second, so every 2-year-old dog is about 24. After that, add about 4 a year for small dogs, 4.5 for medium, 5.5 for large and 7 for giant breeds. A 7-year-old medium dog is about 47.
Is one dog year really seven human years?
No. A 1-year-old dog is nearly grown and can have puppies, so it is closer to a teenager than a 7-year-old child. After the first two years, small dogs age more slowly than seven a year and giant breeds about that fast or faster.
Why do big dogs age faster than small dogs?
Large and giant breeds tend to have shorter lives and reach their senior years sooner, so each year counts for more of their lifespan. Researchers are still working out exactly why size and lifespan are linked this way.
What is the 16 ln(age) + 31 formula?
It comes from a UC San Diego study of Labrador retrievers by Wang and colleagues, published in Cell Systems in 2020, that compared age-related DNA methylation in dogs and people. Take the natural log of the dog's age in years, multiply by 16 and add 31. It was built from one breed and is not meant for young puppies.
When is a dog considered a senior?
It depends on size. The 2019 AAHA Canine Life Stage Guidelines define the senior stage as the last quarter of a dog's expected lifespan, so a giant breed reaches it years earlier than a small one. Your veterinarian can tell you when that is for your dog.
Last updated October 17, 2026.