How we calculate your dog's age
Our calculator translates your dog's age into human years using a two-stage model that reflects how dogs mature explosively early in life and then age at a pace set by their size. This page shows exactly how the math works.
Every number below is an estimate. It gives a realistic picture of which life stage your dog is in, but individual health, weight, genetics and care always affect how a particular dog ages.
The model in three steps
First year ≈ 15 human years
The first year is an enormous developmental journey. A puppy goes from completely helpless to an active, nearly mature dog in twelve months. We therefore count the first dog year as about 15 human years, regardless of breed.
Second year adds ≈ 9 years
The second year finishes off maturation. We add around nine human years, so a two-year-old dog broadly matches a 24-year-old human. Up to this point the curve is nearly identical for all sizes.
From year three: a fixed yearly amount, set by size
After two years the curve flattens, and each new dog year adds a fixed number of human years: roughly 4 for small breeds, 5 for medium, 6 for large and 7.5 for giant. This is why large breeds 'catch up' and overtake small ones over time.
A worked example
Let's calculate the age of a five-year-old Labrador Retriever. The Labrador is a large breed, so we use the addition of 6 human years for each year after age two:
- First year
- 15 human years
- Second year
- + 9 human years (= 24)
- Years 3 to 5 (3 × 6)
- + 18 human years
- Total
- 42 human years
So a five-year-old Labrador corresponds to roughly a 42-year-old person. A small breed of the same age would land at around 36 human years, because it only adds 4 years per dog year.
Dog years to human years by size
| Dog's age | Small | Medium | Large | Giant |
|---|---|---|---|---|
| 1 year | 15 | 15 | 15 | 15 |
| 2 years | 24 | 24 | 24 | 24 |
| 3 years | 28 | 29 | 30 | 31.5 |
| 5 years | 36 | 39 | 42 | 46.5 |
| 8 years | 48 | 54 | 60 | 69 |
| 10 years | 56 | 64 | 72 | 84 |
Figures are rounded human years. The age charts on the breed pages use exactly the same calculation as the calculator.
What the model doesn't capture
- Genetics: some breeds and lines are predisposed to conditions that can affect lifespan.
- Weight and diet: excess weight strains joints and organs and can noticeably shorten life.
- Exercise and mental stimulation: active dogs often stay healthier for longer.
- Care and prevention: dental health and routine vet visits catch problems early.
Why this is an estimate
The model builds on the size-based approach used by, among others, the American Kennel Club, and on research showing that dogs age very rapidly early in life. It's a solid starting point, but not an exact science. Two dogs of the same breed and age can be in completely different shape. If you're ever unsure about your dog's health, your vet is always the best source.
Want to try it on your own dog?
Pick a breed and age in the calculator, and we'll do the math for you.
Open the calculator