Medium dog age in human years
21–50 lb (9–23 kg)
Medium breeds sit close to the average for all dogs, adding about five human years for every calendar year once they are past two.
Medium dog age chart
| Dog's age | Human years |
|---|---|
| 1 | 15 |
| 2 | 24 |
| 3 | 29 |
| 4 | 34 |
| 5 | 39 |
| 6 | 44 |
| 7 | 49 |
| 8 | 54 |
| 9 | 59 |
| 10 | 64 |
| 11 | 69 |
| 12 | 74 |
| 13 | 79 |
| 14 | 84 |
| 15 | 89 |
| 16 | 94 |
The first year counts as 15 human years and the second adds 9 more, which is common to every size. From the third year on, each calendar year adds 5 for a medium dog.
What counts as a medium breed
Whiskerclock treats a dog as medium at 21–50 lb (9–23 kg). That covers Beagles, Cocker Spaniels, Bulldogs, Welsh Corgis and Border Collies, among others. Weight rather than height is the usual dividing line, and breeds near a boundary can reasonably sit either side of it.
Life expectancy and life stages
In a UK study of 584,734 dogs, medium breeds had a median life expectancy of 12.5 years, matching the median for the whole population. Applying the 2019 AAHA Canine Life Stage Guidelines definition of a senior dog, which is the last quarter of expected lifespan rather than any fixed birthday, puts a medium dog in that stage from about 9.
Both figures are approximate. Life expectancy is a population median drawn from UK veterinary records, and the senior age is a rounded figure rather than a threshold — individual dogs vary widely, and no published guideline sets a fixed senior age by size.
Against the “multiply by seven” rule
The old rule of thumb has no research behind it — the usual explanation is that someone divided an average human lifespan by an average dog lifespan and the number stuck. For a medium dog it errs in both directions. At one year it gives 7, against 15 here, badly understating a dog that is already fully grown. The two lines cross at about age 7, and from there the rule runs ahead: at 15 it claims 105 human years where this chart gives 89.
Why size changes the arithmetic
Larger dogs live shorter lives, and the reason is not simply that they die of different causes. Comparing 74 breeds across more than 50,000 dogs, Kraus, Pavard & Promislow, The American Naturalist, 2013 found that the strongest effect of body size was on the rate of ageing itself: mortality risk climbs faster in bigger dogs as they get older. Size therefore changes the shape of the curve, not just where it ends.
Sources
- McMillan et al., Scientific Reports, 2024 — life expectancy by body size, 584,734 dogs.
- Kraus, Pavard & Promislow, The American Naturalist, 2013 — the effect of body size on the rate of ageing.
- 2019 AAHA Canine Life Stage Guidelines — the definition of the senior life stage.